When you say "I love you", it means that you love him/her in a such romantic way. It's from the more deep of your heart, cause it's not graduated like "un peu, beaucoup ou bien" ( a little, a lot or as well). It's like a declaration. But, if you say to someone that you adore him/her, it means that you're taken by a sort of impetuous crazy feeling, in a sense of litteral excitation, like an ephemer passion which is quivering all.

If you say "I love you" to your friend, you rather say: Je t'aime beaucoup. = I love you Je t'aime comme mon meilleur ami(e)! = I love you as my best friend! *But in the casual conversation(especially when you making a joke) you can say "Je t'aime" or "Je t'adore". If you want to express your feeling strongly to your lover, you can also say: Je t'aime tant tant=so much/very much

The same in English you may say "I love you" or "I adore you" depending on your feelings.

Je t'amie>Je t'aime beaucoup, mais on ne dit pas "je t'aime plus", parce que ca signifie aussi "je ne t'aime plus."

I like Je t'adore~~~ :D