My boyfriend is contacting me less frequently. What gives?

I've been dating this guy for a few months, and everything seems to be going well between us. We are already exchanging "I love you" and we text and talk on the phone every day. When we first started dating, we would email each other from work every day, sometimes 2-3 times a day. Over the course of the last two weeks, he has been acting the same (affectionate and happy to see me), but he takes longer to reply to emails during the day, sometimes not replying at all. My friend says this is a sign he is losing interest, but I don't think it's a big deal since we still talk on the phone every day and see each other several times a week. Should I ume that he is just getting comfortable in the relationship and simply phasing out daytime emails because we communicate in other ways, or should I be alarmed by his change of habit in general? He says things have been really busy at work and I believe him. My friend is convinced that this is a bad sign, because guys seldom change their habits in a relationship unless they are unhappy.