I agree 100% with your last line ;). OK, my statement "trying to make someone else happy, will never ever fail to make you happy", perhaps needs a footnote such as "in moderation, obviously. does not include sacrificing your entire life for someone determined to remain unhappy or suffering from severe depression". Then again, its surprising even someone with depression who doesn't give a lot back, you can still go around thinking "well I tried" and your friends respect you, perhaps they emulate you. Good begets good, and all that. That's the idea of the pay it forward movement etc
I can guarantee that there exist several data points that refute this.
Definitely can guarantee many cases where doing this to excess for some people is a net negative.
Can certainly show cases which led to divorce (i.e. always trying to make spouse happy was unstable, and the marriage would have been much better off with randomly ignoring spouse).
Helping others can make you happy. Just don't do it at the expense of self care.