I first read this wonderful and evocative absolute and utter gem of a story at around the age of twelve (and it was likely one of the first longer novels I read entirely in English, not counting those books read entirely for school). And I simply adored Frances Hodgson Burnett's The Secret Garden when I read it as a young teenager (or rather, a tween), I continued to love it when I reread it multiple times while at university, and I still massively loved the novel when I reread the story for the Children's Literature Group in 2011 (and I much continue to love it, having reread it at least twice or so since then). And indeed I honestly do think that I have actually enjoyed The Secret Garden even more as an adult than the times I read the novel when I was younger (and that is definitely saying an awful lot). For when I first read The Secret Garden as a young teenager, I was certainly much enchanted by the garden (and of course, the Robin), and really liked and enjoyed reading about the Sowerbys, but I did kind of consider both Mary and Colin as somewhat too spoiled and selfish (I understood their problems and indeed felt empathy, but I also felt more than a bit annoyed at and by them, something that I certainly did not experience as much during my adult rereads). Because as an adult reader, I actually and firmly believe that most, if not even all of both Mary's and Colin's problems and behavioural quirks (be they emotional or physical) were and are the result of parental abandonment and emotional neglect (maybe even abuse). They act and react towards the world the way the world (or at least how most of the world) has always acted and reacted towards them. And without the garden, but also without characters like Martha, Susan and Dickon Sowerby, without Ben Weatherstaff and the Robin, there would never have been any change in and for Mary (or at least, not ever enough change), and by extension, there would never have been any change in and for Colin and his father either.
Now one interesting and thought-provoking fact presented in The Secret Garden is that there actually seems to be a real and almost palpable absence of nurturing father figures throughout (except maybe Dickon, but he is just a boy and in many ways resembles more a Pan-like nature deity, and Ben Weatherstaff really is too old and curmudgeonly to be considered nurturing and fatherly). We do have quite a number of nurturing mother figures portrayed who aid Mary, and later Colin in their recovery (Susan and Martha Sowerby, and even Mary later becomes somewhat of a motherly and nurturing figure towards Colin), but we never see or hear much about a Mr. Sowerby (he is a complete nonentity). And while indeed much is made of the fact that Mary Lennox' mother did not seem to want her child (a fact that is rightfully criticised), that Mr. Lennox did not trouble himself much about his daughter either, while mentioned briefly, is also seemingly accepted as an acceptable societal given. Also that Mr. Craven has spiritually and emotionally totally abandoned Colin, and cannot stand to even see him when he is awake just because his son's eyes supposedly remind him of the boy's dead mother, while this is indeed noted in The Secret Garden, his rather vile and nasty attitude and behaviour towards Colin, towards his son is not (at least in my humble opinion) subject to nearly the same amount of harsh criticisms that Mary's emotional and spiritual abandonment by her mother is. And while I do realise and even understand that the death of Mr. Craven's wife was traumatic for him, both Mr. Carven's and Mrs. Lennox' actions, or rather their lack of love and acceptance towards their children have had the same horrible psychological (and psychosomatic) consequences, basically turning both of them into emotional cripples, and Colin into a hysterical hypochondriac who thinks he has a crooked back.
The Secret Garden clearly and lastingly demonstrates that children (no that anyone) can only show love, can only be lovable, if they have experienced love themselves. In the beginning of the novel, Mary is described as tyrannical, unpleasant, thoroughly "unlovable" and also as somewhat odd. But how can Mary know anything about love, if she has never experienced love? Her parents certainly do not seem to want her, and she has basically been abandoned to the care of servants, who have also been instructed to keep Mary out of the way as much as possible (and in her innermost soul, Mary likely also realises this and much and rightly resents this). Mary's temper tantrums towards her Ayah and other servants, her desire to always get her own way, are not merely Mary imitating the behaviour she witnesses among the ex-pat community in India (although that likely also has a major part to play). I believe that in many ways, the servants also act as representatives of her absent parents, and by lashing out at the servants, Mary is also lashing out at her careless, unloving, absent parents by proxy.
And even when Mary first arrives at Misselthwaite, there is still a real and ever-present danger that she will never be able to change, to emerge out of her shell (or to change enough, for at least in England, Mary has the opportunity to go outside and play/run, which was not possible in India due to the hot, stiflingly humid climate), for many of the inhabitants of the manor, but especially Mrs. Medlock and Mr. Craven regard Mary, or seem to regard Mary the same way that her parents did, either not at all, or as a cumbersome, even loathsome burden. And without Martha, Dickon, and the influence of Martha's mother (Mrs. Sowerby), and of course, Ben Weatherstaff and the Robin (who is a bird, but might just represent the spirit of Colin's deceased mother), not much would likely have ever changed for Mary or within Mary. There might well have been some physical improvement of her health, but her mental health, her soul, would likely have remained for the most part sour and disagreeable and stagnated.
Finally, I do have to admit that I have a bit of a problem with the fact that oh so many of the adults portrayed in The Secret Garden (and even inherently positive individuals like Martha and Susan Sowerby) keep bringing up the fact that Mary's mother was supposedly very physically attractive, and that in many ways, Mary is often judged negatively because she is plain, while her mother was considered very beautiful. However, Mary's mother does not in any way care about or for her daughter, and had, in fact, never wanted a daughter, and in my opinion, her careless, unloving attitude (and that of her husband as well) is reflected in Mary's countenance, her whole being. Thus, even though Mrs. Lennox might have been physically sweet looking, she basically has a careless and unloving and massively sour (read nastily ugly) soul, which is in my opinion reflected in her daughter (both spiritually and physically).
And just to furthermore point out that this here "Norton Critical Edition" of The Secret Garden (which seems to have been published in 2006) is to be most highly recommended, especially for anyone interested in both the novel (the narrative) and its historical contexts, diverse critical voices etc., as it provides not only the text proper (which is simply and utterly magical, of course), but also much supplemental information and materials about Frances Hodgson Burnett and her timeless literary classic. And although I do not think that this edition lists every piece of extant literary criticism on The Secret Garden, there truly and fortunately is a goodly amount presented, as well as a solid, although not extensive selected bibliography (most definitely a more than adequate starting point for serious academic study and research).