Submit interesting unique links and post more substantive comments (maybe add a paragraph or two your previous comment) and you will be able to downvote soon enough. But I don't understand the appeal, when I do it is usually accidental (really easy to do on a tablet)...

I usually downvote comments that are derailing the thread or are so lazy as to feel like a waste of my time to have read.

I'll often upvote comments with opinions that are counter to my own, especially if they are presented in a clear, coherent, or novel way that causes me to think a bit.

Downvoting is a gated power on this site?

You just need 500 karma to downvote.

It helps prevent brigading and ensures that users have a moment to adjust to the community and its general expectations before being able to participate in community moderation.

If a user cannot reach that low bar after a while, they probably aren't a particularly active or positive member of the community.