I have learned the importance of being a happy missionary. Happiness is a choice and even if things don't go our way, we can still choose to have a good day. When others see how happy we are, they want what we have. A happy missionary baptizes and an unhappy missionary doesn't. We can't share how wonderful our lives are because of the gospel, if we don't show that we are happy!
So be happy every day! It's a lot more fun that way, too! That beings said, I still believe there is such thing as being too happy....so keep it in control! LOL