But happiness at the expense of someone else doesn’t last for too long.
A very simple example. I’ve heard many many times two people who mutually decided to be together and then later down the road, one person backs out for whatever reasons. Now, the other person in this situation has been hurt bc he/she has dedicated his/her time, feelings, part of life etc. and this decision will probably impact him/her for a very very long time. Isn’t this being selfish? How come not enough thought was put into making a decision in the first place and giving the situation proper thought to avoid hurting someone?