What would happen if one made thoughts the sole aim of one's life?

"if" is a pretty famous poem. It's one of the first poems I actually copied out for pleasure. In reference to your query, I think Kipling is trying to extoll the virtues of control over thought as opposed to simply giving in to emotional impulse,

"Or being lied about, don't deal in lies,

Or being hated, don't give way to hating.."

This is Kipling's moral compass on how to be a man. Reflection and control over one's thoughts are what builds respect among others and, most importantly, one's self.

"If you can think - and not make thoughts your aim.."

Thought is not the destination, it is simply the means to get there. Simply acting on thoughts not reflected on leads to a life that lacks wisdom and maturity.