This is one of my favorite stories. It was told by Joseph F. Smith in the April 1900 General Conference for the Church of Jesus Christ of Latter-Day Saints.
"My mother was a widow, with a large family to provide for. One spring when we opened our potato pits she had her boys get a load of the best potatoes, and she took them to the tithing office; potatoes were scarce that season. I was a little boy at the time, and drove the team.
When we drove up to the steps of the tithing office, ready to unload the potatoes, one of the clerks came out and said to my mother, "Widow Smith, it's a shame that you should have to pay tithing."
My mother turned upon him and said: "William, you ought to be ashamed of yourself.
Would you deny me a blessing?
If I did not pay my tithing, I should expect the Lord to withhold His blessings from me.
I pay my tithing, not only because it is a law of God, but because I expect a blessing by doing it.
By keeping this and other laws, I expect to prosper and to be able to provide for my family."
I believe in this. Every time I do a good deed, or pray for someone, or answer a call for help or otherwise look out for and help someone else, I'm not doing it because I'm this great person. I'm doing it because I know that one day I will be in some sort of trouble, and when I am I will expect a blessing. Following in Christ's footsteps is the only way I know that will 100% guarantee my happiness.
It's like taking out an insurance policy with God.