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Doing something nice for a "selfish" or self-serving reason doesn't cancel out the fact that you did a nice thing. Unless you tell them, nobody knows your motivation behind doing the nice thing.
It makes no difference to them.
Bring home-baked goods to a gathering so people compliment your food and tell you how incredible at baking you are. Donate to charity to impress people. Smile and be kind and polite so people like you. Compliment people in the hopes they'll compliment you back. Help someone pick up their things because it makes you feel like the main character. Let a stressed parent with crying children cut ahead of you in line because you just want the loud kids to be gone as soon as possible.
People don't know why you did it.
People do know what you did.
People know you brought home-baked goods to a gathering. Charity got a donation. People know you were smiling and kind and polite to them. People know they got a compliment. People know someone helped them pick up their things. The parent knows you did a kind thing for them.
Whether you do it selflessly or self-servingly, your action doesn't change, nor does its impact.
Not doing nice things because you "just want something in return so it's selfish" is worse than doing nice things for self-serving reasons.
Putting good vibes out into the world because you want to pretend you're a dashing and chivalrous young lad whose wealthy family tragically passed leaving you to inherit the mysterious family mansion and all the men admire you and all the ladies talk about what an honest, good young man you are and all the nonbinaries get gender envy from you and the unlabeled and agender folks just think you're really cool and everyone attracted to men fancies you is good and harmless and fun.