I don't know... Maybe the sea, with its spectacular coral reefs and fishes. Maybe the golden mountains all around Dahab. Maybe the small tents on the beach inviting you to laziness. Maybe the people of Dahab. Or maybe all of them in the same time. Or maybe other things which escape on a first sight.
All my friends are referring to Dahab as "that small little crazy place". Crazy? Maybe... Depends on each one point of view. Little? Definitely not. Dahab village might be small, but there are lots of places which lays down on the coast up to 20 km from the village. As Ras Abu Gallum, natural park and life sanctuary (15 km north from Dahab). Or Sid Gabber, south, where it says to be the best spots for diving around.
You don't need to come to Dahab only for diving. The corals could be seen within few meters from the shore snorkelling only. But diving it is much more better. And lots of fun...
"Unfortunately nobody can be told what Dahab is. You have to see it by yourself." A movie stereothype. But hides a real truth.

Have fun!
Mike