The Mass Effect 3 ending choice isn't really a choice when you think about it.

 Okay, so, for Mass Effect 3 you've got 4 choices for the ending.  Destroy.  Control.  Synthesis.  Reject.

The Rejection Ending is stupid, because all civilization will be wiped out, and all advanced sentient life destroyed.  What have you been fighting for through three games if not to stop that from happening.  One VERY IMPORTANT thing to note when choosing this ending, however, is that when you choose to reject all of the Star Child's options, HE SPEAKS WITH HARBINGER'S VOICE.

Which leads me to the Synthesis Ending.  This is the ending that the Star Child pushes the hardest.  This is the ending that HE wants you to choose.  HE, as in the kid WHO SPEAKS WITH HARBINGER'S VOICE.  This is the choice that HARBINGER wants you to choose.  Whatever Harbinger wants, I pretty much want the opposite.  This is a bad choice, because it is the choice that your bitterest enemy wants you to choose, which brings up questions over whether it will really be what he says it is, or if it will just be another way of indoctrinating and enslaving all life in the galaxy.

So we've got two choices left.  But Control isn't looking all that great either.  Rewind five minutes and see how that went for the Illusive Man.  You're going to control the Reapers by taking the place of the Star Child, are you?  And there's no way that you're not going to be indoctrinated and start the cycles all over again once you've been overwritten like you've seen so many others throughout the trilogy?  Yeah, no thanks.  Didn't work out too well for the Illusive Man, and there's no guarantee that you're not going to end up a Reaper puppet just the same.

Destroy is really the only choice here.  It's the only choice that removes the threat forever.  It's the only choice where you can be sure that you have actually won, and the Reapers will never return to enslave and destroy ever again.  Yes, the cost is high, but can you afford NOT to pay it?  Peace with the Reapers is not possible.  They are pure evil that cannot ever be reasoned with.  They must be removed from the equation completely, so Destroy is the only path that makes sense.  This is probably why the Happy Ending Mod defaults to this ending, because the guy who made it understood everything I just said.