There are many great reasons for studying the martial arts. Self-defense, the thrill of competition, and personal development are only a few of them. However, there is one thing which should not be your chief goal: to become an all-powerful war god (or goddess).
Most people who have this goal would not frame it in those terms. That being said, when you hear what they want to accomplish, it is apparent that this is what they are truly after. If someone wants to defeat Jon Jones or Ronda Rousey with both arms tied behind their back, clear out a biker bar with their bare hands, or take out a team of Navy Seals with their pinky, they really don't want to study the martial arts. They want to become a god and have chosen self-defense as their path to deity.
Maybe your fantasies aren't that extreme. Maybe you just want to be able to manhandle a 22-year-old Olympian...and you're a 45-year-old white belt. Maybe you want to show those street toughs who hang out in front of your apartment building who's boss...and you're a 100-lbs. college girl who's only been to one women's self-defense seminar. The fact is that having unrealistic expectations of what the martial arts will do for you will cause you problems. Best case scenario, you quit when training does not live up to what you saw in Jet Li films and miss out on all of the benefits that self-defense provides. Worst case scenario, you put yourself in a dangerous situation after a week in the dojo and get yourself killed.
No amount of martial arts training will make you invincible. No matter how many black belts you get, you can still be beaten by someone who knows how to exploit your weaknesses. Not to mention that bullets are faster than your hands, baseball bats are stronger than your legs, and knives are sharper than your elbows and knees. Also, if a bunch of thugs jump you on the street, they aren't going to line up and fight you one-on-one like they do for Bruce Lee and Jackie Chan on the big screen.
So whatever your goals are, make sure that they (and your training) are realistic. You will not only be more likely to become a successful martial artist, but you will have more fun as you do it.