The clutch plates are simply sticking together because there is no reason for them not to. In a car, when you use the clutch, the plates actually separate. In a bike, when you pull the lever, you are merely making room for them to separate if they want to. And that separation isn't much more than a fraction of an inch between the plates. After the bike has been sitting and the plates have been pressed together by the clutch springs for hours or days, they naturally grow fond of each other and hold each other tightly, what with being all lubed up in all that oil and everything. (don't get excited) When you pull in the lever, the driven and the driving plates spin merrily along in unison waiting for a reason to disengage. That reason comes in the form of you selecting first gear. Now the driven plates must stop and await further commands while the driving plates continue spinning. That, my friend, is the clunk that you feel and hear. The reason it doesn't happen during the ride is now obvious, no? Look at a drawing in a service manual and see if you can pick up the concept. Better yet, step outside and yank the clutch out of the bike. :-)