For me it's the desire to constantly be around that person and enjoy their company. Even if it's for a stupid reason like "can I crash on your couch for a nap?"
And wither when they're not around. And have fluffly feels inside when I daydream about us.
And feel Pain In The Chest ™ when they have more fun with someone else than me.
And a dagger of jealousy stuck in my back when they speak about their crush.
But mostly it's just a silly happiness to have their attention, and simple things like going for a walk feel like a magic moment.