I tend to be less particular about texts than some composers. I know many composers who don’t involve the commissioning ensembles at all in selecting the text, or who retain final veto, or who control the text decision in some other ways.
I personally think of myself as a hired craftsman whose job it is to work on behalf of the client who has commissioned me. As such, I am happy to work to set the text that the commissioner has in mind. (I nevertheless give myself permission to say to a client, “I don’t think I can set this text; perhaps you want to consider a different text, or else I can suggest some composers who might do a better job than me with this text.)
What about when it is my choice? If there’s a commissioning ensemble, I’m going to work as hard as I can to understand them and their intentions for the piece: that will help me select an appropriate text. Beyond that, it’s a combination of collecting texts (i.e., reading a lot) and instinct.
I’m at a moment where I’ll be selecting some texts in the near future for some exciting new projects. So I’m working hard to revisit believed poetry (public domain only) and seeing what speaks to my current moment.