Because other people reward you socially for doing things for other people. If you do something good for person A, it makes sense for a person A to reward you -- they want to reinforce the behavior they benefit from. But it also makes sense for an unrelated person B to reward you, despite not benefiting from this specific action -- they want to reinforce the general algorithm that makes you help other people, because who knows, tomorrow they may benefit from the same algorithm.

The experimental prediction of this hypothesis is that the person B will be more likely to reward you socially for helping person A, if the person B believes they belong to the same reference class as person A (and thus it is more likely that an algorithm benefiting A would also benefit B).

Now who would have a motivation to reward you for helping yourself? One possibility is a person who really loves you; such person would be happy to see you doing things that benefit you. Parents or grandparents may be in that position naturally.

Another possibility is a person who sees you as a loyal member of their tribe, but not a threat. For such person, your success is a success of the tribe is their success. They benefit from having stronger allies; unless those allies becoming strong changes their position within the tribe. So one would help members of their tribe who are significantly weaker... or perhaps even significantly stronger... in either case the tribe becomes stronger and the relative position within the tribe is not changed. The first part is teachers helping their students, or tribe leaders helping their tribe except for their rivals; the second part is average tribe members supporting their leader.

Again, the experimental prediction would be that when you join some "tribe", the people stronger than you will support you at the beginning, but then will be likely to stab you in the back when you reach their level.

Now, how to use this knowledge for your success in the real life. We are influenced by social rewards whether we want it or not. One strategy could be trying to reward myself indirectly -- for example make a commitment that when I make something useful for myself, I will reward myself by exposing myself to a friendly social interaction. Second strategy is to find company of people who love me, by using "do they reward me for helping myself?" as a filter. (Problem is how to tell a difference between these people, and those that reward me for being a weak member of their tribe, and will later backstab me when I become stronger.) Third strategy is to find company of people much stronger than me with similar values. (And not forget to switch to even stronger people when I become strong.) Another strategy could be to join a group that feels far from the victory... a group that is still in the "conquering the world" mode, not in the "sharing the spoils" mode. (Be careful when the group reaches some victories.)