Because love cannot be kept hidden for long.
Sure, love is the greatest of all the feelings. Afterall, everything a person does is somehow motivated by love. And when people tend to express their love for each other, they often become passionate. At times, even more than necessary. But hey, who doesn't like passionate lovers. But only as long as they are yours. The moment you witness such PDA's, you get that 'Ewww' mode on all the while wishing that it should have been you. Don't lie now, come on.
Anyway, these are few couples who took it to the next stage.
Not cool people, not cool. Why do you have to rub it in the faces of poor singles, huh?