What did MJ mean?

Well...

When u fall in 사랑 당신 get butterflies. That person is all 당신 think about. It's like being HIGH. it's the most wonderful feeling (like a high)

However... When you're in love, and that loved one theta hurt 또는 hurts you, it hurts your very and 당신 cry

That's the difference :)

i would say 'falling in love' is the start...u start falling in 사랑 and then 당신 are in 사랑 and thats 'being in love'
maybe he means it makes him high to fall in 사랑 but when he is in 사랑 he cries? IDK! hahaha its just a song 'high' and 'cry' rhyme ...