The law of diminishing returns needs no introduction. Your second slice of pizza is less satisfying than your first. Your second million dollars is less valuable to you, personally. If you think econ 101 has any life wisdom to impart, it deserves being elevated to the status of “life heuristic” - especially because, unlike some other concepts in econ 101 (comparative advantage anyone?) it feels true in a boring way, not in an actively counterintuitive way.
It applies even in increments that feel “too small to matter”. Brushing your teeth once a day is still vastly better than zero times a day, even if you’re missing toothpaste - you’re still mechanically disrupting the cavity-forming biofilm and making your teeth last that much longer. Or doing 10 minutes of cardio a day is still vastly better than doing none at all, even if 20 mionutes still feels like a burden you’re not willing to take on.
There’s a natural reverse of this: Most of the time, people who promise you increasing marginal returns are selling you snake oil. Claims to the contrary are at least mildly extraordinary, and require explanations beyond the usual - network effects, compound interest, or something like that. It happens: First mover advantage + networking effects paid off handsomely in the case of Facebook, and it made business sense for them to push aggressively in that direction. But most of the time, in daily life, the law of diminishing returns is and should reign supreme.