1) Your mood will improve
When you feel good when someone else is successful, you simply feel good. These good feelings are what you need in order to accomplish your goals soon, according to an article on the addicted2success.com website.
2) You ego will not be too inflated
That's always a good thing. Even when you're currently at the top, the bubble of success can pop suddenly. Don’t become egoistical. Feel good about others too.
3) You will have more friends
It's true that everyone likes people who make them feel good. Praise others. Smile at them when they tell you their achievements. Not only will you be more popular, you may also get some inspiration. You may find role models to follow.
4) You will see that success is not a zero-sum game
Even if you find it painful, celebrate other people's achievements. The more times you do, the clearer it will be that there's enough success for everybody. Be excited for others and suddenly you'll step onto the road to your own success.
-jk-