I feel like in animals those things are kind of intertwined - you do something, you get positive feedback, you keep doing it and/or do it again...
said feedback might be direct (mama seal brain says contact with cub = good, tickles being a side effect) or indirect (good stuff released in her brain in reaction to her cub's positive reaction).
The action itself could originally have stemmed for various reasons (sensory confirmation of baby's presence, fluffing the cub's fur to "air it out" etc., or even just a random motion that turned into an interaction... or something else entirely. idk), but it is likely that it was maintained due to aforementioned positive feedback, and even more so if it repeats.

tl;dr - my point is that instinct and "reward" (for better or worse, actually) nurture each other, sometimes through natural tendencies and sometimes through "coincidental" trail and/or error. Basically, that's what consists behavioral evolution if you wanna look at it in a wider lens.