Aggression is the red-headed step child of human traits. Everybody just wants to beat it and call it names without taking the time to get to know it more intimately.
Aggression has many forms and disguises. That guy with 700lbs on his back squatting, grimacing and grunting. That nerd tucked away in the non-fiction corner of the library reading with papers strewn about the desk, who's there every day until closing. The professional that is racking up certificates and P.H.D's building his portfolio. The perfectionist at work that won't stop a job until it's perfect in their eyes. The mother tearing a new hole in some idiot for putting her child's life in danger. The boxer that is throwing flurries or the mma fighter going for an arm bar. The guy typing at his computer spewing out examples to make his point. All these people are exhibiting a form of aggression. Aggression is not necessarily a bad thing, aggression drives us to better ourselves, to change the world, to re-shape what is into something better, to accomplish things.
Some things in life don't just fall into your lap, some things you have to pursue. Throughout time people who knew how to direct their aggression down a certain avenue where successful beyond what one might expect. I doubt anybody hasn't heard of Sigmund Freud (who tirelessly hit the books, experimented and aggressively ass kissed his way to become a renowned neurologist), Genghis Khan (who really isn't as bad a guy as one might think, he did have some noble goals, he may have been a touch mean though), and Hitler (he aggressively worked his way up through countless political circles to get to where he was, he was an extremest but you can't deny he was successful) just to name a few.
In my humble opinion, if your looking for success in anything you should do it aggressively, with focus and clarity. It won't fall from the sky so quit looking up and buckle down. If you want to lose weight or gain strength or muscle hit the weights, the track, circuits, whatever you do do it hard.