An optimistic person thinks the best possible thing will happen, and hopes for it even if it's not likely. Someone who's a tad too confident this way is also sometimes called optimistic.
If you see the glass as half-full when others see it as half-empty; if you look on the bright side of things, you're optimistic. If the chain falls off your rusty old bicycle, a tire goes flat, the frame cracks down the middle, the seat keeps twisting around, and you say, "But look! The little bell still works — I'm sure this bike will be fine," you're being very optimistic, though some might venture to call you "delusional."