It is sometimes used like “lucky” because people who have no idea what it means get it confused with “fortunate” and use the two interchangeably.
If you want to use it to mean “lucky” it should be in the sense of “a purely accidental thing that happened to be good”, not just any good thing that happened.
If I travel to a foreign city on a whim and meet my future spouse there, that might be fortuitous. (It would also be fortuitous if I travel on a whim to a foreign city, get hit by a truck, and end up paraplegic.) If I train for years and finally manage to achieve my dream of running a marathon, that is not fortuitous.
Your link offers the definition “Happening independently of human will or means of foresight; resulting from unavoidable physical causes.”
If you want to use it to mean “lucky” it should be in the sense of “a purely accidental thing that happened to be good”, not just any good thing that happened.
If I travel to a foreign city on a whim and meet my future spouse there, that might be fortuitous. (It would also be fortuitous if I travel on a whim to a foreign city, get hit by a truck, and end up paraplegic.) If I train for years and finally manage to achieve my dream of running a marathon, that is not fortuitous.
Your link offers the definition “Happening independently of human will or means of foresight; resulting from unavoidable physical causes.”