Learn Russian: Почему ты не хочешь соглашаться?

After a negated verb of desire like не хотеть (to not want), Russian strongly prefers the imperfective infinitive. Because the person doesn't want to do the action, it isn't going to happen, so Russian focuses on the general idea of the action (imperfective) rather than its successful completion (perfective).

Why does соглашаться end with the reflexive particle -ся?

While -ся is often used for reflexive actions (doing something to yourself), it also acts as a marker for verbs that don't take a direct object (intransitive verbs). You can't "agree" an object, you just "agree." Many Russian verbs require -ся for this reason without meaning "oneself."

How does the meaning change if I drop не хочешь and just ask "Почему ты не соглашаешься?"

Dropping не хочешь makes it a simple factual question: "Why aren't you agreeing?" By including не хочешь (don't want), you highlight the person's reluctance, stubbornness, or refusal to yield. It translates directly to "Why don't you want to agree?"