Realistically when it comes to human emotions, you cannot just say something once, and then expect another person to not want to talk about it again.
example: (ISTJ to ENFP: I told you I loved you three weeks ago, why do I have to say it every night, when we talk? Don't you believe me? )
"I love you" followed by several converstaions about "How was your day? The weather is nice today? how is school? work?
It's like what Jennifer was saying, you need to 'stoke the fires.' and repeate yourself every so often, it doesn't undermine you, it makes the other person feel loved.
I personally listen for patterns, if I'm expecting someone to say something then when I don't hear it, I sense something is wrong.
Personally it's common preference among most human beings in a relationship.
Its nice to be told you are loved, and appreciated several times a week, to keep the other person aware of how you feel about them. Its hard to just go by actions. To ENFPs, words are very powerful.
We like honesty more than anything.
I like it when ISTjs show affection through back rubs, or asking how Im feeling, what im thinking. I find them to be good listeners.