Why do baby's get dehydrated?

Not enough water in,: Too much out, or both. Can happen with diarrhea, vomiting, diabetes, overheating, overexertion, fever, or just plain not drinking enough.

Small volume: Babies get dehydrated because simply they are so small. Fever causes evaporation, and babies are all surface area comparatively. Thus a moderate fever causes decent water loss. Couple that with vomiting and some diarrhea and a lot of fluid loss happens quickly. Babies cannot be reasoned with that they need to "just drink more", so all that water loss leads to irritability, not thirst.