As so often in this sermon, the words of Christ are similar, but not quite the same as in the Sermon on the Mount. There are variations constantly.
But we are to imitate our Father because we desire to be like him. We desire that the attributes we most admire in him should be in us also. Mercy is not natural to fallen human nature; it is a trait found only in those who are born again. The Pharisees were incapable of it, and chose to disregard its importance, and to focus instead on what brought them the praise of men. Mercy does not expect any reward. It does not act because of anything in the recipient to commend them. It acts because of the satisfaction of doing what is good freely.