Why do people not afford the same courtesy for others that was done for them?

This has been a question on my mind for a while. I just do not understand how some people can go about their business without seeing that they have taken advantage of another. What I mean here is simply the swapping of roles. It's like if you are always there to listen to someone vent, but the other person never lets you vent to them. See how unfair that is? That's the sort of thing I mean. At the same time this is about a different situation, but the idea is still the same.

There has been a simple role reversal between myself and another person. I love this person and think they are the best. The thing is though is that the other is not doing for me what I did for them. I understand the other's position as well. What I don't understand is why they cannot either just do what I did for them, and was expected of me, or simply let me know that simply cannot so that I don't have to be disappointed whenever they fail me. Maybe I am being too sensitive about this, but at the same time I know they can relate to the position I have now. So what gives?

Am I asking too much of this person to simply do what I did for them? Am I expecting too much in thinking that they ought to do this simple thing for me? I may never know. What I do know is that I feel that this is unfair and do not like it. I am only asking for a little time to be dedicated to the task that I ask to be done. I did the same thing for this other person when they were in my position. For them to simply blow me off is a little irritating. Maybe I am being petty in writing this up, but this has been a thing that has been bothering me. I know other people have experienced this sort of thing so I am hoping that at least those who relate can feel some sort of validation.

If this has made you, reader, think about times you have either been in my position or the other cool. I am not trying to make you feel bad either. I just want people to know that this is just not a cool thing to do. It is really a bit disheartening actually. Makes me feel like my work isn't worth being repaid. There is no equivalent exchange going on. For Fullmetal Alchemist fans that was for you. You, I also feel, will get where I am coming from. I gave my time and energy for this person and they are not doing the same for me. Very unequal I'd say. But this is just my opinion. Hopefully, some readers will relate. If not, oh well.