"Kryptonite" is an allusion to the world of Superman, a superhero who is invulnerable, except when he is close to a metal called "kryptonite" from his home planet Krypton. Saying that something is your kryptonite means that it makes you all weak and you cannot defeat it. Saying that dogs are your kryptonite is a way of saying that they disarm you completely. It would sound exaggerated, but it would make sense. :)

Yes, these days, it makes sense! Ok..it's like this.... For Superman (yes, the superhero), Kryptonite makes him become weak. It can be used to talk about something that causes you to become weak (or soft-hearted). Now, you try and use it in your own sentence!!! :) Enjoy learning English! Thanks, David