[I_]

Grass is faster, meaning for any given shot over the net you have less time to respond. This encourages attacking behaviour.

Clay is slower, meaning even off fast, angled, difficult-to-reach shots the ball bounces higher and slower, meaning you have time to get to it, thus prolonging rallies and points.

But beyond that, it's not just that players choose to play more aggressive on grass - a given shot is more aggressive on grass.

This is why the serve is more powerful on grass - you open the point with a powerful shot, and on grass that's harder to return.

See rally length differences here: https://twitter.com/i/web/status/1246341537753006082

[seabass-labrax]

Fascinating! Thanks for the explanation.

[harry8]

Note Rafa has also won wimbeldon on grass, against Federer at his unbelievable peak (in the midst of winning it 8 times), in 5 sets, in the final, finishing in the dark. And won again 2 years later to put those trophies with his 14 French Open titles.