Interferes with sense of smell.

Dogs with pointy ears can get wet ear waxy buildup if the ears are left wet or damp.

if it happens to be raining cats and dogs it might cause a bit of raunchy dog-on-dog action. maybe it's just not in the mood.

Do YOU like walking in the rain? I used to enjoy it, under certain circumstances...... Nice gentle rain, relatively warm, and a good bottle of wine....... My dog, on the other hand..... If she neeeds to go, she'll go out in the rain. If I throw the ball, doesn't matter what the weather is, she'll happily play. But, barring that, she'll just sit in the doorway, and watch the rain/snow fall.