It could be poverty in some cases, but I think the majority will just be inherently dishonest people who steal things because they can. I think it makes them feel smart to 'get one over on' other people and companies.
Look at the other thread about stealing from hotels (and other establishments) and there are a number of people admitting to (or reporting others) deliberately thieving towels, bath robes and other items that are most clearly not consumable that are expected to be used up/taken away.
All of these are people who can afford to stay at hotels, so poverty isn't a factor - they just have a natural selfish streak that means that, if they can get away with it, they think they have a right to do it, and will.
I used to find it weird when banks put their pens on chains - and made them quite difficult to actually use - but they obviously had to do that because of the surprising number of lifestyle thieves who will not only steal a pen worth a few pence, but in so doing, mean that there are no pens left for future people who need them. Same with public toilets that now have lockable holders that only let you pinch one little sheet out at a time; imagine being the kind of person who would steal toilet roll, just because it's there and you have no morals.
Anybody trying to justify it as 'just a pint of milk' should realise that they are potentially depriving a housebound person or run-ragged parent of a lifeline. It could easily be an exhausted mum with babies and pre-schoolers (possibly disabled) who then either have to go without their breakfast or otherwise it takes her ages to get them all ready and go out to the shop to buy more, which she might be ill-able to afford, so they have to go without elsewhere. All of their lives made so much more difficult because of a selfish thief who thinks it's a laugh to steal, just because they can get away with it.
Not to mention the milkman/woman's livelihood, which will be severely affected if enough people cancel them as a direct result of doorstep milk thieves.