People on MN (sorry, yes inspired by another thread) seem to get very upset about being taken advantage of and there's lots of advice like "no is a complete sentence". People get very upset if they feel they're being put upon etc and I can see that if it's all one way but I've never experienced that.
Very early in my career I attended a (very 1980s) seminar, something like How to Win Friends and Influence People. I don't remember much of the content but one thing that really stayed with me was the power of the favour.
It has served me well, whether it's making the coffee, contributing to a report that wasn't really my job, picking up lunch for colleagues when I go to get mine, staying late to explain a complex task a colleague is struggling with, collecting others' children from school, sharing research I've done that can help with someone else's presentation, giving an acquaintance the benefit of my professional expertise, watering a neighbour's plants etc etc.
Being in a position where there are always people you can ask for help because you did it first makes life a whole lot easier and happier IME, even if it does mean that some people "owe" me more than I owe them. Plus it makes for a warm feeling inside and really is there any harm in being others' "go to" person?
But so many people here are determined that no-one should be "allowed" to take advantage. Surely it is these things that make the world go round? Does it make you any happier to know that you haven't given anyone more than they've given you?
Sometimes it's about men taking from women, but I worked the first 23 years of my career in a very male dominated environment. I was often the only woman in my department and I found the same thing there. I helped them, they helped me.