When we truly feel the love of God and have a sure knowledge that God lives, we want to share that with others.
Understanding of God's love comes in stages. First, we understand it theoretically. Then, we understand that God loves us, values us, and wishes us well--we understand that we need God. We understand His love as a child understands a parent's love. Finally, we begin to understand that love means sacrifice. As we slowly begin to understand Christ's infinite Atonement, the unbounded sacrifice He made for us and the daily sacrifices He still makes in serving us, we simultaneously begin to serve and bless those around us as well. (The reverse is just as true--the more we serve, the more we can understand Christ.) The love of service and sacrifice has potentially infinite power.
Testimony works similarly. At first, we believe because others tell us it is true, and it seems to make sense. Later, we receive our own spiritual experiences that confirm it. Finally, we may receive the grand spiritual gift of a burning knowledge and desire to share the gospel. Before my mission, I had a strong testimony, but it did not begin to burn within me until I specifically asked for that blessing. I find that it slowly wanes unless I continue to thank the Lord for it and ask for it to continue.
Ask, and ye shall receive. This is specifically applied both to charity (Moroni 7:48) and to testimony (Moroni 10:3-5) in the Book of Mormon.