I hear it all the time. It follows, 'You're an angel!' and precedes a little self-indulgent laugh.
That's not what kept *me* from fostering, back when I was kept from fostering. (We'll get to other reasons people who love animals don't jump in.) But I'm finding it's a hugely popular sentiment.
Yes, it's really, really hard to love a dog and then let him go. There are fosters whose leavetaking I have mourned for months.
But the fact is, if I didn't take in those dogs, and no one else took in those dogs, those dogs would be euthanized. Way I look at is it's a helluva lot better for me to be missing some fantastic canines who are now with others who love them than for me to not get involved, and my heart to be unbruised, and those fantastic canines to be dead.
Sometimes it's not easier -- for me, the person, who lives, regardless--but it's always better.
"To close your eyes will not ease another's suffering." ~ Chinese Proverb
And another thing. You *wouldn't* keep them all. Some of them are difficult. Some are difficult before they are fantastic. And as much as you care for them, you remember the difficult times (which may not, in fact, be completely behind you) and wish the dog the very, very best in her forever home.
Besides, you value your space. Are you going to stack dogs floor to ceiling like some crazy hoarder? Because then, yes, donating to your local rescue group or shelter would probably make more sense.
And by letting one go, you get to save another.
The fact is, the dog (and cat--I mean cats too every time I say dogs) you foster is going to a screened home, to a person or people who love him and have been waiting for him. If you hadn't taken this dog into your home, giving him a place to feel safe and calm and bloom, and then taken him to that adoption event, it's possible that his Forever Family wouldn't ever have met him. And his FF has been waiting for him.
I know, because the people who adopt my fosters email me and tell me so. They also say they can't imagine their lives without him. And they send pictures.
So yeah, you can let 'em go.