system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why chocolate is bad for Dogs?
gpt
The main poisonous thing to dogs in chocolate is a chemical called Theobromine.  The chemical is actually poisonous to humans as well, but in addition to the fact that we weigh quite a bit more than dogs, we also break down the chemical much faster than dogs do, so it doesn't have as much time to build up in our systems.  Also of note is that the more "Cacao Solids", sometimes written on bars as the percentage of cocoa, the more theobromine; so white chocolate is relatively harmless to dogs, moving up to milk chocolate, then dark chocolate, and finally baking chocolate as the most dangerous, as it is almost entirely cacao solids.

If you're curious as to how the chemical is harmful; it is very similar to caffeine and causes similar adverse effects: nausea, increased urination, irregular heartbeats, and if large enough amounts are consumed, even internal bleeding, heart attacks and death.  Hope that helps.