I have got a glimpse of what Joshua Harris is trying to say when he mention:
Dont get into relationship because of selfishness but to truly love that person

In a Christian perspective:

Selfishness is when we rush things out in our timing before waiting for God response. Because we are leaning on our own desire (the "i want it to happen right now attitude") and will end up hurting that person & our-self even if we met the right person but its on the wrong timing.

Love is when we truly sit down and pray to wait for God's answer. Moreover, knowing that human love is weak, to love God above all first is also a way of you to love that person, because you know a relationship will only be at it best when God is at the center of it.