We are doing this because we are Christ-followers. Jesus went about doing good and taught his followers to do the same.
By doing good we hope folks will ask, "Who are you and why are you doing this?" giving us opportunities to share the love of Christ in words as well as deeds.
Sharing Christ is our ultimate motive, but not an ulterior motive. We are not trying to manipulate people into becoming Christians ... we are doing this because we are Christians.