Oh, I play with friends too. But often these friends introduce less than desirable elements as players, friends of my friends who does not really make great roleplayers. And then what do you do? If you tell the newcomers to leave, you insult BOTH the newcomer AND your long time friend. If you let them stay, they wreck you GMing efforts.
And when friends want to introduce new players, or friends want to return to your gaming group after a 10 year absence. They always get mad when they hear that 'no, I have too many players already'.
I already rotate groups, and quite frankly I think my current players get too little playing time already.
One last tip:
If a player approach you with his new found love, and he wants to include her with the group, think VERY carefully:
1. Chances are that she does not at all care about roleplaying and just want to control what her boyfriend does. (been there, done that).
2. Chances are that her ideas concerning roleplaying does not match yours. When she suddenly demands to have a Dungeons and Dragons character in the middle of a Call of Cthulhu adventure, don't be surprised. (been there, done that).
Oh I have a dragons hoard of absurd examples, suffice to say that it is going to be near impossible to get rid of such a player, without causing severe strains on your friendship with her boyfriend.