Look for someone who is more selfless than selfish. If a relationship is going to last, you will want someone who is capable of putting someone other than themselves first. Everyone is selfish to some degree but is your lover willing and able to put his or her interests on the backburner at times? Does your partner primarily display a me-first attitude or do you both take turns in giving priority to the other?
There should be ample evidence of this, on both sides. And by the same token, no one should ask someone to do something that he or she isn’t willing to do.