Joy is usually pictured as something we go looking for, but here it simply turns up. The speaker is going about their day when, "out of nowhere," a feeling of happiness rushes in and takes hold of them. Nothing in the poem explains why. There is no cause, no good news, just the sudden fact of the feeling.
The most interesting choice is the image of joy's "tiny paws." That turns the emotion into a small creature, something like a kitten or a little animal that leaps up and grabs hold. "Seized" sounds forceful, almost like an attack, but the paws are tiny, so the grip feels playful rather than frightening. The speaker seems startled and delighted at once, caught off guard and not minding it.
The mood is light, quick and a little surprised. The poem is only five short lines, and that brevity fits its subject: joy is a brief, unexpected moment that arrives and takes over before you can prepare for it.