Why are decomposition reactions called the opposite of combination reactions?

A decomposition reaction is the breakdown or separation of a chemical compound into its elements or simpler compounds, while in combination reactions two or more elements or compounds combine to form a single compound. That is why decomposition reactions are called the opposite of combination reactions.

Decomposition Reactions:
CaCO₃ → CaO + CO₂
NH₄OH → NH₃ + H₂O

Combination Reactions:
N₂ + 3H₂ → 2NH₃
CaO + H₂O → Ca(OH)₂