A quotation by Alfréd Rényi
If I feel unhappy, I do mathematics to become happy. If I am happy, I do mathematics to keep happy.
Quoted in P Turán, "The Work of Alfréd Rényi", Matematikai Lapok 21 (1970) 199 - 210.
A mathematician is a machine for turning coffee into theorems.
[Rényi's colleage Paul Turán is credited with the refinement that "weak coffee is suitable only for lemmas".]