Who says you should? You don't have to.
On the other hand, even from a selfish viewpoint, if you had a choice leaving someone better off or worse off for equal investment from our part, it is naturally better to leave someone better off;
A positive impact on someone else is more likely to return positive results in the future (or no results), whereas a negative impact is more likely to return negative results (or no results).
Even if there is no chance that positive impact will yeld positive effect for the future, there is often a chance that a negative result may yeld a negative effect. Since for an easier and better life it may be preferred to minimize the negative effects in the future, it is therefore better to take the path that capitalizes on minimizing the negative results.
Since relationships generally deal with intangible investments, it may be often hard to determine which outcome of situation (i.e., positive/negative) will require additional investment, and a positive outcome may even generally require more investment, however since the goal may be to avoid negative effects on the future, it may be indeed better to leave everyone better off than worse off, since it is less likely it will come around to bite you in the ass.
Think of going to a dentist. I'd happily pay 20$ for routine checkup for them to confirm that all my teeth are healthy. There is no real positive return in such scenario, since I leave with the same teeth condition as I came in, however, this may prevent a negative scenario from developing, where I'd have to fork out 500$ to get my teeth fixed with a side of painful experience. Of course, there's always possibility that my teeth will be fine even if I don't do checkups, or that my teeth will rot no matter how many checkups I do, however, on a general scale, if you get your teeth checked routinely, you will be less likely to end up with an anus for a mouth.
Just my view on this from most rational standpoint.