There's lots of ways that helping others can potentially make you feel happy. It can confirm your opinion of yourself. If you think that you're a good person, doing good things gives you a nice confirmation high. It can work the reverse as well, doing good things when you think you're bad can help to alleviate your guilt or shame.
Seeing the effect your helping others has can also be rewarding. It gives meaning to your actions. Makes you feel like you have a sense of agency.
The dynamics are important though. It's about balancing the effort and rewards. Helping someone who is ungrateful is unlikely to have the same effect. Whereas, helping someone who is too dependent can be stressful.
If helping people is causing you unhappiness, then maybe you need to either help other people or take a look at why you're offering help. It could be that you need to revaluate your expectations.
Also, being selfless is not really a great way to be. You have to have some regard for yourself on whether you need to help others or not.
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