I do not know what is up with me + lineation these days, and I should be working on my own thesis or reading someone else's rather than writing this post. Ah well. Factors that go into line breaks:
1) form (e.g. certain number of syllables or beats, end-rhyme)
2) breath/phrase/clause breaks (places one would naturally pause slightly or where there is a discrete clause. breaking across those can create greater momentum and sometimes a jarring sensation, which can be okay/desirable in some circumstances but generally not, I think)
3) making the first and last words of the line interesting and/or significant. We pay most attention to the words at the beginning and ends of lines, because of orthography, structure of English, left to right reading, etc.
4) similarly, placing more emphasis on a certain word than there might be otherwise (in unlineated writing or in a lineation that more closely followed breath or phrase breaks)
5) evoking different or multiple meanings (keeping in mind productive vs. nonproductive ambiguity, that is, ambiguity that actually enhances meaning or expands things in some useful or meaningful way, rather than just cramming in extra connotations/weirdness for the hell of it)
6) surprise or irony
So yeah. A few years ago I seemed to be more into the latter 3, especially doing the unproductive ambiguity thing. I was so all about trying to make poems and lines that made the reader feel like the back of their head was blown off (however ED phrased that exactly). Now I'm really not. That is one flashy trick out of a whole bag of them, and there are better ones. Now I feel like I'm focused more on the first two, or more than I was earlier at least. I don't want to be breaking lines just because it's "poetry" and there are line breaks. One poem recently I was like, fuck it, it will be a prose poem. I don't know, line breaks are awkward for me now.
Oh, also I forgot 7), how it looks on the page. This is an irritating one.