Why do some people cross themselves and others don't?
Making the sign of the cross has been part of Christian life from the first centuries, and is practiced in several Christian faith traditions. Some Christians trace a small cross on their foreheads; others mark a larger cross by touching their foreheads, then their breastbone, then one shoulder and the other. Sometimes people do this using all five fingers, sometimes using the first two fingers and thumb (representing the Trinity). There isn't one right way to do it.
The sign of the cross can be used for different purposes. We often use it when we mention the Trinity (i.e., "In the name of the Father, and of the + Son, and of the Holy Spirit"), when we remember the promise of our baptism, or after communion. Luther suggested that every Christian should start and end each day with the sign of the cross. Using the sign of the cross reminds us that we belong to Christ, that our true name is now Child of God, that we are one with Christ in dying and in rising.
Different people have different reasons why they use the sign of the cross. When I was new to seminary, I had very little experience with church and didn't know anything about the sign of the cross, other than that Roman Catholics and baseball players seemed to use it a lot! I had a strange desire to make the gesture, but had no idea what it meant. So I asked a classmate why he used it, and he told me, "When I mark the cross on my body, it reminds me that, because of Jesus' gift in baptism, I carry the power of Jesus' death and resurrection in my person, not only for my sake but also for the sake of the whole world. Everything I am is marked with this gift, and I'm called to live it out--not just as an abstract idea, but as a physical, real thing. Making the sign of the cross helps me remember that." That explanation made sense to me. I started using the sign of the cross immediately, and have used it ever since; it expresses a truth that's important to me that's hard to put into words.
Not everyone feels the same desire to use this gesture, and there's no requirement to. I use it; my husband doesn't. One of our children uses it, the other doesn't. Whether or not you use it depends on whether you find meaning in it.