Many people begin karate in their youth to avoid being bullied. I have heard this both in modern times and I have even read that was the reason Kyan Sensei's father had Master Kyan begin training. Being able to protect one's self and being able to protect loved ones in one of the most prominent reasons to train in karate over the years. If this is the case, remember this in your training. Ask questions that relate to these types of situations so that you can utilize your training.
Many other people train in karate because they want to get/stay in shape. This is also a very wise choice since unlike many sports, karate practitioners can continue training well into their twilight years. Karate also is adjustable to your body type. It's not like some sports where you're either built for it or you aren't. If you look at Osensei's teachers, you'll notice that while they were all very healthy and in shape, they also all had very different builds. If this is your primary reason for training, make sure that you are including enough physicality and hojo undo in your daily training.
Some students take up karate because they want to win trophies in tournament situations. While this isn't what our particular style was designed for (our style existed years before the first tournament), practitioners of Shobayashi Ryu have long been great competitors in tournament play (see Tammy Marino, Joe Lewis, etc.)
Hopefully all karate-ka have this on their list of reasons to train: It's fun! Enjoy yourself as you train. Have fun with it and enjoy the connections you make with your dojo brothers and sisters.