Zoro was a man of actions over words. True, he was often heard giving rather epic one-liners after dealing the finishing blow on his opponent. But that was in the heat of the moment. When it came to the more complicated uses of the human language, like giving comfort and inspiring people and making plans and the like, he was often as literate (and tactful) as a brick.

There were many things in his mind that he wanted to say, and he might've blurted them out during dinner, had his Captain not broken down like that. And now, holding tightly to the heartbroken bundle in his arms, he couldn't think of a single thing to say that would make it better; he could only make Luffy feel worse, feel even heavier with guilt.

Ace would have never forgiven you for giving up your life like that.

You promised that you would become Pirate King. Are you going to take that back?

What is the Strawhat crew without their Captain?

We're not heroes, remember? Heroes sacrifice their lives for people. Pirates don't.

They were all statements heavy with implications, heavy with the suggestion that Luffy had failed (but he didn't), had forgotten (but it's ok).

Anything he said would probably only make a new wound on Luffy's heart, which was already a shredded, pulpy mess as it was. With that in mind, he resolved to say nothing, instead acting as a large, human-sized handkerchief. After all, it was fine to cry it out every once in a while. It certainly had worked after his first miserable defeat by Mihawk. Come to think of it, Luffy had also been there, his deceptively easy laugh assuring Zoro that the future Pirate King still had absolute faith in his swordsman. Zoro didn't have the confidence to laugh like Luffy (it would have probably come out as a condescending growl), but this time the tables had turned. This time, he would witness Luffy's tears turn into new resolve. And as Zoro clutched Luffy slightly tighter, he sealed up everything he wanted to say, including what was probably the honest, selfish truth:

You can live without Ace.

But we can't live without you.