It was because Simon Pegg and Nick Frost always have a real life joke about loving each other. Each thing they do together they always have to have it implied that 'something' is going on but is never actually 'fulfilled'. Shaun of the Dead, Spaced, etc. Rumours say that this time, they decided to make it WAY more blatant, and not have any female love interests at all, because they *like* having their characters being implied to be interested in each other, and thought it was about time they went a bit further there. lol.
I love those two. :D Can't you just imagine them sitting around going 'And our characters- they can fall in love! but we won't actually say it in the movie! It can all be hugely implied! So that people think it's hilariously funny and don't freak out!'