Here's why:
* Decided is the past tense of the verb "to decide".
* Decided is also the past participle of the verb "to decide".
Adding "ed" to "decide" creates the past tense and past participle forms of the verb, which are used in different contexts:
* Past Tense: "I decided to go to the park."
* Past Participle: "He had decided to stay home."
Therefore, the word does change when you add "ed" to "decide".