Curiosity is its own reason.
Time is what the clock says.
Everyone likes me, yet nobody understands me.
My scientific work is motivated by an irresistible longing to understand the secrets of nature not by other feelings.
[Asked about a book in which 100 Nazi professors charged him with scientific error.] Were I wrong, one professor would have been quite enough.
Solutions to tough problems should be made as simple as possible - and no simpler.