In the Bible lands culture, to constrain a person means to ask him over and over at least three times. If you were to ask me to come to dinner at your house it would not be polite for me to immediately accept your invitation.
The first time you asked me, I would politely refuse and give you a good excuse for why I could not come.
Then you would tell me how you really wanted me to come and ask me a second time. Again, I would give you a good excuse for why I could not make it.
If you really wanted me to come you would ask a third time. Upon the third request, I would know that you really wanted me to come and I would accept your invitation. This is called constraining in the Bible lands.
The love of Christ manifest in his death and resurrection constrains us to live for him. We are reminded over and over of his love.