In my experience there are 2 big problems in that, and they are caused because the view of what is good and what is right and retribution.
In human nature people in fact care way more on what they have to fight to conquer than what they get easy. and after they get used to having something for free, they complain if you charge something.

So, back to the question, 2 things happen:
1- you are too nice, extremelly reliable sacrifice yourself for others, and what do you get? Nothing! People used to your easy nice behaviour will think they don´t need to invest on you, you will see that at work everyone gives you all the tasks they don´t want to do thenselves BUT at the same time the ones that get promoted will be the "dificult ones" as they take you for granted happy where you are, so you don´t need to get raises, to be valued, they dont feel you require any investment to get the best of you, you give it for free... you are the friendzone guy.
2- You get tagged as the do it all guy, and once you are tagged you cant change, you are forced to keep doing it at all times or punishment will be harsh! If you decide to change, to stop the abuse, people will be "disappointed" if you refuse something, you will receive cries of "you was a good person but you changed" and "now that we was thinking about you you disappoint us". (and maybe be fooled that they was really thinking good of you and you destroyed it - they really want to make you feel as the one to be blamed) - on the friendzone relationships is when the loved one makes the nice one feel awful for making the wrong move on the wrong time (but in fact he never had any real chance, this is just punishment for not being the usual puppet)

So if you are the extremelly nice person, try to change it, in real life nobody values saints, they are just explored until depleted and then spit out when they don´t serve anymore.