Pre-Lily days, he would've said he was acting like a girl. But James has since learnt that there is nothing weak about women.
Lily Evans is better than him in every way – she is funnier, and smarter and (he hates to confess) a lot more attractive. Well, maybe not every way: her Quidditch skills are somewhat lacking. Despite this, she is wonderful. She is endlessly fascinating, and his words get caught in his throat the same way the sun gets caught in her hair.
There are days when she frustrates him so much, when her sarcastic quips and dry humour drive him up the wall. There are times when he can tell that he had better leave her alone, too: those moments when his relentless teasing seems like the most childish habit there could be. Those days often mean Lily retreates to the library, turning the unread pages of books angrily. For James it leads to ruthlessly scheming with the Marauders. Their pranks have few limits that day, which always pleases Sirius.
This restlessness with each other always passes, and ends in the way a summer storm does – with catharsis. They become reacquainted, both physically and mentally. Her hands run through his hair with a passionate dominance and he desperately tries to count all the freckles that make her bare body look like a mortal heaven. They talk and lie together, and James has to make the choice between gazing at the flyaway copper (an underappreciated metal, in his opinion) of her hair or at her eyes, which were a shade of green that would never be rivalled. He goes with the latter every time. Those eyes will be the death of him. He only hopes that if they ever have children together, it will be Lily's wondrous eyes that are passed on.
These thoughts confuse him. He's slightly ashamed of how much time he devotes to dreaming of their future together, and how many times he forgets quite how to breathe when he sees her. For the first time in James' life, he is seeing the world as artists do: beautiful and raw and terrifying. No, it's not a girl he is acting like... It's Mooney.