‘Constraineth’ translates the Greek term which means something like ‘squeezes us’, ‘presses us from both sides’. And the English word ‘constrains’ means just that: presses us to do something.
Think of the care and the love of Christ; it constrains us, it presses us. Think of the privileges you have had already: conversion, new life, emancipated minds and understanding, strength over sin, guidance, joy and peace in believing, answered prayer. Think of the patience of the Lord, do all to please him.
Think of the effect of your life on others. Some may be saved through you, some may be hardened through careless acts committed by Christians. Think of the blessings yet to come on you, on your children, your family from Christ. How much we owe him.
And think most of all of those heavenly rewards. They are given not on account of what we have done, but on account of what Christ has done for us, and on account of the motions of the Spirit producing good thoughts and acts in our lives. They are rewards that are so gracious and undeserved. How much we owe.