You're not alone! I'm a huge CM Punk fan too but I find myself wanting Brock Lesnar to win at SummerSlam too. It should be noted that I've always been a big Brock Lesnar fan, as well and I've just been waiting for the day that CM Punk and Brock Lesnar would cross paths. I just thought the juxtaposition was too good; the way Heyman would deal with Punk.. trying to calmly reign him in and then with Brock he'd be losing control it was only a matter of time before he would have to play mediator or pick a side. I figured he would pick Brock Lesnar because Paul Heyman's just too good as a heel.
I disagree with you on this being Punk's fault storyline wise, I do think it's more Paul Heyman's fault. By design Paul Heyman is made out to look like the bad guy but in the given situation I do believe it's so good because Paul has some room to stand on; there is some blame that can be thrown Punk's way. But overall, I look at it as Heyman's fault. Punk suffered three devastating losses in a row one to The Rock, one to John Cena for the #1 contendership (first time Cena has actually beat Punk too) and one to The Undertaker. Initially before Paul Bearer died, Punk's goal was just to take away The Undertaker's Wrestlemania streak from the fans because his WWE Championship reign was taking away from him. After not being able to get the job done, Punk walked out to have a break.
It was Paul Heyman that dragged him back at Payback because Chris Jericho coaxed him into it; just like how Stephanie McMahon and Triple H coaxed Paul Heyman into agreeing that Brock Lesnar would face Triple H at SummerSlam last year. CM Punk never asked for the match with Jericho, and really didn't even have to show up at Payback and face Jericho and it would have all been on Paul Heyman. Heyman almost cost Punk the match at Payback, and Punk simply requested some space, from Heyman. Looking at it like that from a storyline perspective (a.k.a. nerdy wrestling fan analysis) I think the blame goes on Paul Heyman for doing what's best for Paul Heyman not what's best for CM Punk and then just being a vindictive, worm; we all know and love Paul Heyman to be.
However, the main reason why I want Brock Lesnar to win at SummerSlam is, because I've just been loving the CM Punk / Paul Heyman banter. I can't get enough of it; as much as I loved them together, I have to say the promos we've seen over the last two weeks have been amazing. And I just get excited thinking about all the things they could do in the next couple of weeks. Leading to SummerSlam. I don't want to see this end on one match. In addition to that, Brock Lesnar since returning to WWE has had a horrible track record. I still can't fully wrap my head around why Brock lost his return match to John Cena, and his loss at Wrestlemania to Triple H. He's only had four matches since returning to WWE and he's lost half of them, so I would like to see Brock pick up the win against Punk now to solidify Brock more as a monster; WWE does a great job billing Brock as a beast, but when it comes down to it, in a high pressure situation he's usually lost the match.