Motivation is so much stronger...

...when you feel competition is there, and could potentially take something away from you.

And you therefore need to complete things quickly!

Somehow, not sure exactly how, this also relates to Parkinson's Law of complexity.

It seems that with competition, you also force yourself to simplify the task (because you have less time), thereby reducing the complexity and allowing you to complete the task faster. Thereby competition also makes you do more in less time, in addition to the motivation.

So as always, competition is a great thing - that is, IF you want to accomplish many things here in life and develop yourself! Do you?