#3 Be prepared for heartbreak. While this undefined relationship makes you happy *besides, you like him a LOT, think you are falling for him, and you can change his mind later on*, it comes with a lot of complications. A man who cannot be open or honest to you about his feelings can be difficult to deal with now and, most likely, in the long run.
After all, only a small child cannot articulate his feelings or make up his mind and stick with it. If he refuses to attach strings to make you his girlfriend and you’re okay with that right now, then be prepared to get your heart broken and your self esteem trashed sooner or later.
Being with a man who doesn’t want a relationship will leave you frustrated. Chasing after him when he made it clear he doesn’t feel the same will destroy your self esteem and open you up for more hurt. If you’re sure you have an invincible heart of steel, then go ahead.