This is interesting because I think I have a habit of doing this sometimes... by 'doing this' I mean spewing my idealistic plans all over the place. I think often what I'm looking for is not what any of you have mentioned- not reassurance that I'm right, or an addition to my ideas. I think most times, ironically, I'm looking for an argument.
So it's strange, but it's like here is my grandiose, unattainable, yet utterly genius idea. This is probably how he feels: he's got an ingenious plan to save humanity. He knows that it can't truly work but he feels like he can bounce the idea off you and you ought to bring him down to earth. Then he can argue with you fervently to protect his idea, secretly reconsider, and come up with a revised plan, or a new one.
And, if it were me, your response also indicates your level of interest. So right now he must be upset because your passive agreement shows a low level of interest (which is obviously true). He's telling you his passions, and you are saying yeah ok that sounds nice call me in the morning for tea. He wants you to be excited even if it means disagreeing, especially if it does.
I feel like I can really relate to this one, hope it's helpful.