This is a waltz-time love song. Musically, it's along the lines of 'Sixteen Candles"... or "Earth Angel," one of the songs that inspired Simon to take up songwriting altogether.
The song repeats the opening words "Ask me why" over and over to start lines, in a rhetorical device (a.k.a. "speechwriter's trick") called "anaphora." One famous example of this is from Winston Churchill: "We shall fight on the beaches... we shall fight in the fields... we shall fight in the hills..."
It's really a tender, touching song of young love, never falling into schmaltziness-- until the very end, when we get a "big finish."
"Ask me why I go sleep/ Counting the days I spent with you/ Instead of counting sheep," it starts. The other things the speaker does is "pray" they will be together forever, "thrill" to her touch, and "hold" her "so tight."
Other effects she has on him are that he is "filled with a heavenly bliss/ Each time we kiss," and that he feels that, when he's not with her, "it seems like time is standing still/ Each minute's like a year."
Yes, this is puppy love at its purest and most sincere. He tells her "it's so hard to say goodnight," which implies that he does, in fact, do so at the end of their dates.
Oh, and why does he feel the way that he does? " 'Cause I love you!" Yes, not "Because," but, pointedly, " 'cause." So, they are likely still in high school.
Is this a love sonnet worthy of William Shakespeare or Elizabeth Barrett Browning ? No. Is it every bit as heartfelt? Yes.
Or, perhaps more accurately: Yeah.
Next song: I'm Lonely