Get our your lucky rabbit's foot! Fortunate is lucky, but fortuitous means by chance or accident. Silly rabbit, these words aren't the same.
Fortunate means having good fortune that you're born with or win somehow. You're fortunate if you're rich, good looking, and in love. Here are some examples full of fortunes:
Let's say you are fortunate enough to have a good job and some savings. (Business Week)
More fortunate refugees have been taken in by local families. (Reuters)
Fortuitous traditionally refers to something that happens by accident or chance — either good or bad. It descends from the Latin forte, meaning "by chance." People often use fortuitous to mean lucky because they want to sound fancy, but you don't have to be one of them. Let's stumble upon these fortuitous examples:
Meetings with other individuals in the course of foraging, basking, or seeking shelter, are fortuitous and have no social significance. (John M. Legler)
There are also fortuitous street finds, like the Eames lounge chairs he discovered near their apartment in Greenwich Village. (New York Times)
Fortuitous has also been used to mean fortunate for so long that the meaning is morphing. Fortuitous is still separate from fortunate, though — it's closer to serendipity. The usage guru Bryan A. Garner notes that this usage of fortuitous is at stage 3 in his Language Change Index: "the form becomes commonplace even among many well-educated people but is still avoided in careful usage."
Don't thank that rabbit foot for your being so fortunate; after all, it wasn't so lucky for the rabbit. But if you're superstitious, it's fortuitous that you found it right before you bought that lottery ticket.