I've recently had TWO experiences where people insisted on paying for my part of things, seemingly because they think they're in a better spot financially than me? Like "Oh, don't worry about it, I'll cover this drink, my treat."
Look, I'm not struggling, I want to pay for my own drink! Just because I have a small house, a family supported by one income, bike everywhere and shop at Goodwill, doesn't mean you need to buy me a drink. Those are the reasons I can easily afford to buy my own drinks!
In both cases I'm pretty sure I'm doing better financially: one friend makes about the same amount but is going to grad school, another has had some recent health setbacks. I shouldn't assume, though...