So you put in tons of hard work for someone, maybe when nobody else would. You try to give them the best you have. But they treat it like nothing, and they give their thanks to someone else.
Have you ever been in a situation like that before?
And some people will make you feel terrible for being upset about it. Some people, often Christians especially, will make you feel like you are being selfish for being hurt. Some of them might also tell you that they don't get upset like that when their overtures are rejected. They tell you that you are giving with "strings attached." That doesn't help. It helps to only embed your negative feelings more deeply, leading to resentment.
But the thing is... it does hurt when you put a lot into someone and they reject or minimize or dismiss your efforts. There isn't much you can do about it hurting. We aren't robots, and it isn't selfish to have it hurt. It isn't having "string attached."
And when people tell you that they never feel this way, stop and take a look. You probably don't have to look too long to see how they struggled (and maybe still struggle!) with having their overtures rejected by certain people in their lives. In other words, nobody is immune to this, and if someone thinks they are then they are lying or blind.
It becomes "strings attached," however, when you can't let go of the hurt. When you hold on to it, and it settles into bitterness, and you need them to respect you and validate your overtures, then there are "strings attached." That way of relating to people is not only sinful, it will not get you anywhere. It will destroy what you seek to create.
That is why the only real solution to having your overtures rejected is to embrace the hurt head on and move through it, letting it go, and continuing to pursue and engage others.