That is the first question of the Westminster Shorter Catechism (that I had to study and reply to before I joined the Presbyterian church as a youth).
I am sure that you all are aware of Charles Schultz’s (1922-2000) personal Christian convictions and that it seeped into his Peanuts strip from time to time. This week’s reruns have CB addressing just this question, but his answer is hardly catechetical.
It seems to me that Lucy and Linus’ responses are rather profound, given CB’s view that our purpose is “to make others happy.” If that is our sole or primary purpose then indeed somebody (everyone) isn’t doing their job. Now on the other hand, I suppose we could say that this is just “the second great command” put in new terms and I would accept that, but it is given primacy in this account. I think the order (love God first then we are able to love our neighbors, and even ourselves, properly) is rather important.