Is it good syntax to thank the reviewer for every comment in peer review?

I'm in the social and health sciences, so etiquette in your discipline may differ (though I doubt it).

I have never done this. Generally, I thank the editors and reviewers for their time and effort in the body of the response letter, but I stick to business in the individual comments. There may be a really exceptional comment by a reviewer that I may thank them for (suggesting a specific citation I didn't think of or identifying an oversight, where addressing it may improve the manuscript immensely). So, basically my individual responses look more like "As suggested by reviewer #1, we have added additional literature about topic x in the introduction...We have revised figure 2 so that it more clearly displays the findings."

Hope that helps!

This is completely unnecessary and, honestly, looks like you're grovelling. Replies to reviewers are mostly there to indicate that you've considered everything they suggested, to draw their attention to the changes you've made and to allow you to explain why you're not making certain suggested changes. By all means thank the reviewer for anything particularly insightful and thank them if the review as a whole was particularly useful, but there's no need to thank them for every single comment individually and tell them that every word they wrote was amazing and changed your life. It just looks insincere.

I would suggest something along the lines of:

Question 1: Please resize Figure 4.

Done.

Question 2: Why was this test performed?

This was to check that the decreased widget performance really was because of interference from space aliens. We have inserted an explanation of this below the description of the test (paragraph 5, page 4).

Question 3: There's a subtle but important error in your argument on page 8, which the referee must have read very carefully to have noticed. And perhaps a suggestion of how to fix it.

Thanks! We've fixed the problem by [doing whatever you did].

It's fine as it is. If the reviewers have been especially critical and/or the responses are rejecting most of the proposed changes and additions, offering so much thanks might sound passive-aggressive. Otherwise, this extreme politeness is not uncommon in any form of editorial correspondence. I suppose authors just don't want to seem like they aren't grateful.