Well what I've gleaned is that in Japanese it's generally a good idea to be not too direct about certain things, love especially. There is a thread if you use the search function where various people (including people who live/have lived in japan) give their opinion on how to deal with the subject.
Ai shiteru 愛してる Is a really really direct way to say 'I love you' and it's one of those things where japanese learners who read a lot of manga can (wrongly) assume it's common place to say it, when it really isn't. People can be put off by being so direct and forward about it, even in a long-lasting relationship.
I honestly wouldn't touch the phrase 愛してる before I'm married to someone. What I've seen to be more appropriate is using (name)が大好き (name) ga daisuki. It literally means something around 'I like (name) very much' or 'I love (name)'.
If you wanted to take it a step lower to the level of a crush, you could drop 大 and just say (name)が好き。 That means something like 'I like (name)' and if you were actually only just crushing on said person or hadn't dated the person yet I might say (and this is drawing on some vague memories so I wouldn't trust me on this suggestion very much) '(name)ことが好き' which (someone please confirm whether or not my thinking is right in this) would mean 'I like (name)' but you're making it known that you like the 'them' that you have in your head, which may or may not be an accurate reflection of their true personality.
Edit: Here's the thread I mentioned
I love you(thread). You'd probably want to go ahead and read this.