Your office is in the more expensive parts of the town. You work long hours and want a shorter commute, this will leave you with spending a lot of money on accomodation.
And when the team goes out for beers, if you want to be a part of the team, you go out with them. And if you're not part of the team, you won't be progressing fast and getting a better salary.
And you soon realise that the clothes you wear make a difference to your career. You buy proper suits, proper shirts, proper shoes, and so on.
And after working your ass off like that all the time, you need a proper break every once in a while. So you take proper holidays in places far away from work. Those cost money too.