This is not to say that we should do something to feel good or to receive something else in return, but that when you associate your actions with positive attitudes (or think positive thoughts and act on them), you will nurture a positive mental state. For example, getting involved in volunteering work helps one to appreciate their own life much more by learning about the difficulties other people have to overcome. Helping someone with an assignment or homework develops your teaching skills, and also reinforces your own understanding and knowledge about something. Helping a friend with renovations might give you a hint or two about the best place to buy certain goods.
If you've ever helped someone and realized what an enjoyable experience it was, then you'll understand what I mean. If you feel like you're always giving and not receiving, then it's because you're either expecting too much from helping people (which should not be your motivation) or that you've got pretty much everything you need (which you should be grateful for). The only thing you can't get by helping people is selfishness, so if you've got too much of that, then you should start helping yourself get rid of some excess by helping others.