No, this question applies not at the beginning of the relationship, but after a few years or more. Sure, you love the person but this is economics and we think at the margin. Why did you say "I love you" right now rather than two minutes ago? I can think of a few reasons:
1. Anxiousness and a desire to reassure oneself in the face of self-doubt.
2. Irritation at the other person, leading to #1.
3. Desire to manipulate the other person by first making him or her feel compliant and secure.
4. Being overcome by suddenly stronger feelings of love, perhaps because of a Proustian reminder.
5. The simple feeling that too long has passed since having said "I love you," presumably combined with the belief that the words are uttered rarely enough to still have potency. You need to signal you are keeping track of such things.
6. The sex was either very good or very bad, see #1 and #4.
7. One has work or chores to do, and is hoping to create a distraction of some kind.
8. To announce that a conversation is over.
Natasha asks whether in a marriage one hears "I love you" more or fewer times than is optimal. We both think "fewer" is usually the answer, although given the low cost of generating the message, and the possibility of reaping gains from trade, it is not entirely clear why this equilibrium persists.