We need more info. Are you a dime? Is your bike out of the ordinary? Were you naked?
Anyway, it's simple curiosity. Similarly when two motorcyclist pass, they nod and check each other out. When I see a the same car as mine, I give it a look to see who's driving it, or if they've upgraded anything. Same applies with biking. I'll scope out a passing cyclist. I'll be curious of several things. Are they faster than me? What kinda bike they're riding. What gear and accessories they might have. Is she hot?