Only four possible answers.
He's not passionate about you OR He's fucking someone else OR He just likes fucking you...as in, that's all he likes about you. OR finally, he got burned in his last relationship and is scared to go down that road again.
It has nothing do with being afraid of commitment or being weak or being fearful of losing his freedom or being just not sure. We know as soon as you open your mouth whether or not we want to date you. How serious we get is entirely up to you at this point. Yes. Our decision was made that quickly.
Guys have no issues with committing to their favorite sports team, musician, car...whatever. So why should we be unable to do the same with you? The answer lies above.