Here's why:
* "Believe" is a stative verb. Stative verbs describe states of being, thoughts, or feelings, and they are typically not used in continuous tenses (present continuous or past continuous).
* "Believing" implies an ongoing action. However, believing is a mental state, not an action.
The correct way to express this thought is:
* "I believe you." This uses the simple present tense, which is the correct tense for stative verbs.
Let me know if you have any other grammar questions!