If you're competitive, you want to be the best. No one likes to lose, but if you are a competitive person, it will be especially disappointing to see someone else win.
People who are competitive like to compete — to find out who knows the most, runs the fastest, can eat the most hot dogs, and so on. Some people are competitive about everything. You'll know them by their constant comparing themselves to others and trying to find out what others have and do — in order to be sure they are still "ahead." Competitive can describe any contest, like a competitive sandcastle-building event.