Ah, that is a well studies thing! People respect usually your privacy and the space you need around you. But when space is scarce and you have to squeeze closer they still try to keep some minimal space intact, like a couple of centimeters. Only in extreme situations people are in full contact and hate it. So when there is enough space and people choose to come close they usually want that proximity. But the motivation can be weird or even evil and cringe too. My own experience is limited but I found people were seeking the protection of another person closer by or they were pickpockets.