I was going to say work on what you love, but that's not really it. It's so easy to convince yourself that you love what you're doing — who wants to admit that they don't? When I think about it, the happiest and most successful people I know don't just love what they do, they're obsessed with solving an important problem, something that matters to them.
Recorded MIT 147th Commencement, MIT News (Massachusetts Institute of Technology), 7 June 2013 source