Learn Spanish (Latin America): Yo me mareo por la altura cuando viajo a la montaña. - I get dizzy from the altitude when I travel to the mountain.

Because this sentence talks about a current habit or repeated action. When you use cuando to describe something that generally happens every time (like saying "whenever"), you follow it with the normal present tense (the present indicative).

Why is it me mareo and not just mareo?

The verb here is marearse (to get dizzy), which is a reflexive verb. If you just said mareo, it would mean you are making someone else dizzy! The reflexive pronoun me shows that the dizziness is happening to yourself.

Why use por in por la altura instead of para?

We use por here because it points out the cause or reason for an action. The altitude is the direct cause of the dizziness, so por translates well to "from" or "because of" in this context.

Why does the sentence say a la montaña (singular) if English speakers often say "to the mountains" (plural)?

In Spanish, the singular la montaña is frequently used to refer to a general mountainous region or landscape. It is exactly like saying you are taking a trip to "the beach" or "the countryside" in English.