Carpe diem, quam minimum credula postero. -- Horace, Odes Book I
The full quotation is carpe diem, quam minimum credula postero, which translates to something like “harvest the day and put little trust in tomorrow.” The common translation is “seize the day” and it implies an active assumption of control over our own happiness. Do that which will make you happy now, because tomorrow may never come.
It reminds of the old Steve Jobs quote "Remembering that I'll be dead soon is the most important tool I've ever encountered to help me make the big choices in life. Almost everything--all external expectations, all pride, all fear of embarrassment or failure--these things just fall away in the face of death, leaving only what is truly important."