1
general::
Phrase(s): by choice
due to conscious choice; on purpose. • I do this kind of thing by choice. No one makes me do it. • I didn’t go to this college by choice. It was the closest one to home.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Deliberately, as a matter of preference. For example, No one told me to come; I'm here by choice. This expression replaced the earlier with choice, used from about 1500.
American Heritage Idioms