Reasons that you could see a thearapist, that would not traditionally be considered mental illness in casual conversation, can include: coping with occupational stress, coping with grief, preparing for anticipated grief, coming to terms with mortality, (often for children)
learning what mortality is, coping with changes in lifestyle (including divorce, marriage, birth of a child, birth of a special needs child, inability to have a child, moves to unfamiliar locations, moves to unfamiliar cultures, loss of religion, loss of a job, release from prison, release from the service), (as mentioned by erikpukinskis) feelings of inadequacy or low self-esteem, ...
Just about anything people may have talked to priests about in the past they can now talk to therapists about.
Now, if you want to define mental illness such that it covers all of these things, and all the other reasons to see a therapist, that is fine. Words are just what you make of them; you can, of course, define them as you see fit.
However, if you redefine mental illness in such a way then I can see really no justification for using the negative connotations that mental illness normally holds to facilitate mock outrage ("Writing off people who want or provide a diamond ring as mentally ill (as many people in this thread seem to be doing) is silly.") Instead of criticizing erikpukinskis for "writing off those people as mentally ill", perhaps you should instead explain why you do not agree with his suggestion that low self-esteem could be a factor.