This is such an interesting and eye opening thread 
I don't give myself permission to look good most of the time, and have spent some time today after find this thread pondering why.
There's a few factors. I come from a normal working class family, not poor, but not wealthy either. My mother always looked nice, was/is slim and pretty and chose clothes which suited her, but didn't wear (or really need) much make up, wasn't particularly fashion concious and spending on herself was definately bottom of the list- she would rather spend money on us, and as a perceptive eldest child I was aware of this.
I was painfully shy as a child and have always and still struggle with confidence. I hate how this is going to sound so bear with me... I have always been slim and have naturally very blonde straight hair, light blue eyes and strangers/family/classmates etc often commented on my looks saying I was pretty etc. It drew attention to me and as an introverted shy little thing I hated it. I've never had a great sense of style anyway, but as a child I wanted to look cool but not too nice so as not to stand out. My mum did her best bless her, but shopping with me was a nightmare as I never really knew what I wanted (I never confessed to her I was worried about looking too nice!). 'She would occasionally say things like 'it's such a shame, you would've looked lovely in that' if I rejected something, which although meant kindly, made me feel worse. My teenage years were similar, wanting to fit in, but found it hard to do so, and wore minimal make up and 'boring' clothes to ward off attention. I always worried that if I went 'all out' to look my best, a) I was bound to get attention/compliments I wasn't able to cope with, b) who am I to be strutting around like I deserve to be there and c) I might get it wrong and make a fool of myself. God this paragraph sounds awful, but I hope some of you get what I mean.
By my early 20s, I had learned a bit of simple style, coping strategies for my low self esteem and I had a job. In those few years, I did spend more time on myself and enjoyed buying new clothes. Usually quite cheap (New look H&M etc), but if I loved something a bit more £, I would buy it. I remember the day I spent £65 on a smart/casual dress because I loved it and I was going out with my bf for dinner somewhere naice that night- I think that was the last time I spent over £30 on myself in one go!
Now I am a SAHM and I'm back to old habits, dithering over cheap t shirts and jeans, trying to convince myself that they are either affordable, necessary, or not too frumpy. I find it hard to spend money on myself, or either much time now I have DD to look after and am not earning (and still don't have the easy sense of style many others seem to). I've not even bought the new bras I rashly promised myself I could have when I finished feeding DD, as six months have passed, I've no idea of my true size and my old ones 'will do' .
My DH is wonderful and will always tell me to buy what I want (within reason) and that I look nice. Problem is, he tells me I look nice when I know I probably don't (he has no style either!) and tells me I look as beautiful in my scruffs as when I've spent an hour trying to dress up- a lovely sentiment and I love him for it, but it doesn't really give me an incentive to bother really.
I really wish I could be one of those effortlessly stylish put together women, but I'm just not.
I realise this has turned into a bit of an essay, sorry
So interesting to read all the other points of view though to convince myself I'm not just lazy!