One must consider the socio-economic reasoning behind the large majority of homelessness cases.
A couple have already been mentioned: general financial hardship (redundancies etc. - studies show a large %age of individuals don't have enough savings to cover them for six weeks) and domestic and general abuse. Both of these cases, the blame does not land at the feet of the homeless solely.
From personal experience, I would have been labelled and treated (by the local council) as 'homeless' last year, but not qualified for immediate/priority support. A revelation at home meant it was simply impossible to stay living there. Under the local government requirements, those estranged aren't highly regarded and as such you're pushed down the [ever increasing] lists for support. In such cases, these are the individuals who end up forming bonds with others on the streets and form mini-communities. Councils refer these individuals to various charitable organisations, but the nature of these organisations mean that support isn't guaranteed and you're constantly moved around. These individuals need stability and familiar faces and [although any support given is greatly appreciated] feel they're better off in these mini-communities on the street, as the bonds are generally very strong as they look after each other (whereas as the charities move you around, you end up living under the wing of other community 'king pins' and face abuse and coercion to other things, drugs, prostitution etc.)
Given how bureaucratic councils' housing programmes are, getting help can be a real nightmare. For example, I recently read that certain councils (iirc, Manchester City) on their 'rounds' only count those actually asleep as homeless; if you're awake etc, you're not considered homeless and thus, the statistics on this issue are poor making the Govt's plans on this poor and not matching the actual level of the problem. Bristol City Council's housing department require you to fill in SIX forms to be put onto a system for housing support. As is always the case, many homeless individuals have had a poor and sub-standard level of education. How can you expect these individuals to fill in these various forms, and with many requiring so much information (various confirmations of ID)? And unfortunately, administrative support that homeless charities offer is drying up. And then there's the issue of funding for these places. If the govt. insists on cutting housing benefit and housing support, I feel we'll see even more people homeless.
As a libertarian, I firmly believe in the self-fulfilling prophecy. If you want to better yourself, then you should go and find and seize the opportunities and make that happen; you shouldn't expect all the opportunities to come and land themselves at you. But giving the dwindling climate, opportunities are too non-existent and we're forcing these individuals into a cycle of deprivation. So excuse them if they use what little money they can get to numb themselves by drinking. Given the cold weather, it's expected that several of the older homeless individuals will not make it through; I hope we can see it past the poor (excuse the pun) image portrayed in this thread, and elsewhere, and consider the reasoning behind these individuals' very unique set of circumstances.