Using "desde luego"
Why does this mean "of course"?
1 Answer
Easy answer. It means "certainly" or "of course"
If you would simply go to the translator on the home page you would have found it, plus many examples in sentences. Many such expressions can be found in the translator. Please use it for simple things like this.
Big Edit!
I saw that I really didn't answer your question. There is no real reason why "desde luego" should mean "certainly" or "of course" It's just an idiomatic expression like "¿Qué tal?" (literally "what such") which we use to mean, "How ya doin'?"
Here's a link to make it easy.
http://www.spanishdict.com/translate/desde luego