sich aufregen (über/wegen)
to get upset
To become emotionally agitated, angry, nervous, or worried because of a person or thing.
Bitte reg dich nicht wegen dieser Kleinigkeit so auf.
Please don't get so upset about this little thing.

to upset / excite
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to get upset
To become emotionally agitated, angry, nervous, or worried because of a person or thing.
Bitte reg dich nicht wegen dieser Kleinigkeit so auf.
Please don't get so upset about this little thing.

'Aufregen' (to upset / excite) is a separable verb made up of 'auf' (up) and 'regen'. The old verb 'regen' means 'to move' or 'to set in motion'. So 'aufregen' literally means to 'stir up' feelings or set them in motion.
Diese Ungerechtigkeit regt mich wirklich auf.
This injustice really upsets me.
The prefix 'auf' is separable and goes at the end in a main clause. In the infinitive with 'zu', 'zu' goes between 'auf' and 'regen'.
Sein unhöfliches Verhalten regt viele Leute auf.
His rude behavior upsets a lot of people.
Distinguish carefully: 'sich aufregen' (to get upset) is reflexive and means that you yourself become angry or nervous. 'Jemanden aufregen' (to upset someone) is transitive and means that a person or thing triggers these feelings in another person.
Ich rege mich über die Verspätung auf. (Ich werde wütend)
I get upset about the delay. (I become angry)
When you get upset about something, the reason is usually given with the preposition 'über' and the accusative. The preposition 'wegen' (+ genitive/dative) is also possible, but 'über' is more common.
Er hat sich furchtbar über die schlechte Note aufgeregt.
He got terribly upset about the bad grade.
'Aufregen' (to upset / excite) can mean both negative emotions (anger, worry) and positive ones (excitement, anticipation). The context makes clear what is meant. The related adjective 'aufregend' almost always means 'spannend' (exciting) or 'exciting'.
Die laute Baustelle regt mich jeden Morgen auf. (negativ)
The loud construction site upsets me every morning. (negative)
to inspire / enthuse
'begeistern' and 'aufregen' both describe strong feelings. The most important difference is the direction of the feeling. 'begeistern' is clearly positive: something brings joy and awakens interest or admiration. 'aufregen' is often negative and then means that someone becomes angry, nervous, or worried. But it can also be meant positively when something creates excitement or anticipation. That is why learners may confuse the words. If you want to say that someone thinks something is great, 'begeistern' is usually more precise. If someone becomes internally restless, gets annoyed, or is very excited, 'aufregen' fits better.

Der laute Streit im Treppenhaus regt die Nachbarin auf.
The loud argument in the stairwell upsets the neighbor.
Der Vortrag begeistert die Gäste, weil der Redner sehr lebendig erzählt.
The talk inspires the guests because the speaker tells the story in a very lively way.