Without taking on new challenges, you will never prosper. You will be stick in a rut, never achieving any better - and eventually you will get bored and restless, and this will actually make you perform worse.
Well this is the case with most people I know and relate to anyway.
Say you are in a job that you are happy enough with, but new opportunities to progress the career ladder arise and you don't take them.
Eventually, those opportunities will stop coming, and you will regret the day you let them slip by.
As you get older in life, you may be will relate to this a little more. Or maybe it's a feeling that young people experience too.
Carpe Diem. Always remember that. Seize the day.