“Let each of you look out not only for his own interests, but also for the interests of others.” Philippians 2:4 NKJV
All Christians are supposed to treat others with humility and selflessness. The focus is not exclusively towards others. Paul does not say, "Never look to your own interests." In other words, we are to put the needs of others before our own, but this does not mean we completely avoid caring for our own needs. It's hard to serve other people when your own life, health, or concerns are in ruin.
It is very easy for selfishness to creep into a relationship and destroy it. Therefore, if selfishness destroys relationships, it is selflessness that builds them.
Selflessness brings out the best in others. And if you start acting selflessly in a relationship, it encourages the other person to change because you are not the same person anymore. They have to relate to you in a new way. Therefore, selflessness does not only transform a relationship, it also transforms the person.
“Don’t be selfish; don’t try to impress others. Be humble, thinking of others as better than yourselves.” Philippians 2:3 NLT
Blessings!