"But with love comes loss, son. It's part of the deal. Sometimes it hurts, but, in the end; it's all worth it. There's no greater gift than love." - Stoick
You knew the time would come. You just didn't know, or you didn't WANT to know, when. Or even how it would come.
The dragons had been saved once again. Toothless had found his own mate, a Light Fury, a sub-specie of Night Furies.
But Hiccup knew he had to let Toothless go. Be happy with his mate. Be somewhere safe, be the king of The Hidden World. It hurt. It hurt so much to let go of his best friend who had been with him for years on years. Who he'd bonded with so much.
And it hurt you just as much when you knew you had to let go of your own best friend, your own dragon. Although it had to be done, although it had good intentions, it didn't mean that it didn't sting and stab at your heart.
You asked to be alone with your dragon before you can let it go, Hiccup allowing it as he knew the pain of it.
So now, here you were, standing in front of your dragon distanced from the others. It pained you way too much.