This only really counts among teenagers (and people mentally stuck in their teens).
Teens are like packs of wolves. The thing they're most afraid of is being alone. Rather than seeing doing things on their own as a sign of independence, they think it's a sign of weakness or that something is wrong with them.
Since they don't want to be the one singled out by the herd, they will do anything to try to belong to a group (or look like they do).
Although, females do look for social proof, and are more wary of a guy who's alone than a guy surrounded by others. If a guy has a girlfriend, he already has another female's stamp of approval.