Almost no one is able to push themselves to their limits without someone else pushing them too.
Think about it: Athletes all have coaches. Athletes are an obvious example of the raw pursuit of excellence (in generally clearly measurable ways). Great artists often have mentors. Great research groups have great PIs (Principal Investigators), and these groups spawn terrific researchers from their labs.
Even the best, most disciplined people will want to flake every now and then. Even the best, most disciplined people will not always identify how they can improve their process, since they are busy in their process.
That’s why you should get advisors. If you don’t have any, watch out. You’ll likely persist in doing something dumb for too long, or at best, you’ll plateau while your competitors keep ascending. Bad.
Finally, not improving feels bad! Getting better – stretching yourself – is ultimately what you’ll find most rewarding. And it’s really hard to stretch yourself without someone helping you along.
(Thanks to Brandon Roy for stimulating this thought. Not this Brandon Roy, by the way. This one.)