Night Furies don't reproduce like most dragons. Normally, dragons choose a mate, build a net, lay eggs and raise hatchlings together. Simple. Night Furies, however, are a purely male species. To reproduce they need to find a mate, dragon or not, female or not, who they trust and share a bond with. When the time is right, they will inject a venom into their mate which changes their body into an almost half-night fury which will allow them to birth and raise the young.
Apparently the time is right for Toothless.