Why are most women selfish these days? A relationship is about making each other happy?

Todays women reward bad behaviour and vice versa. Trying to make a woman happy is where you go wrong, never deprive her the joy of her working to get you!

That depends on your definition of selfish. If you mean that they won't be your sex slave/maid/unpaid PA, then that's because most women have too much self-respect. Some women are genuinely selfish, of course, as are some men. Some people don't also want to sacrifice their own dreams for someone else's which might be considered selfish but is understandable.