I think we can even make a phenomenological case for act flowing from abundance (rather than need). Think of St. Bernard's quote:
The man who is wise, therefore, will see his life as more like a reservoir than a canal. The canal simultaneously pours out what it receives; the reservoir retains the water till it is filled, then discharges the overflow without loss to itself.
The most common human instance would be romantic love. The lover is open, magnanimous, overflowing. As Josef Pieper says, "Only the lover sings." I don't think the lover is compelled or has lost his freedom, he has just become some sort of spring of life.
If that can happen with us then I don't see why it can't happen with God. Indeed the Christian conception of God
as outpouring love finds a close parallel to the example above. But I think Quid's "Why not?" is an important angle too.
If you really want to focus on the time 'before' creation I would offer an admittedly facile illustration. Suppose it's a beautiful Fall night and you're sitting around the fire with two friends, having a wonderful time, perfectly content. At some point it dawns on you, "Hey, this is really great. Why don't we invite more people? We have plenty of beer and room around the fire." So you do, and out of abundance rather than need. That's my deep 21st century theology for you: the Trinity had some extra beer in the keg. There was no one to invite so they created them.