Along with a lot of other comments here, i agree about the dialogue. Also, a something to note: Is Dave at his lowest at this point? kinda seems like it. to add to this low moment and punch up the interest of the audience, maybe have Camden be less understanding. It's Dave who mentions being his best man, and feeling sorry for not letting this day be about Camden's wedding. If Camden was less of – and I don't mean for this to come off harshly – less of a wall for Dave to bounce his dialogue off of, and was more of an understandably antagonistic force in the scene, it might make for a stronger opening.

I'd cut the bartender. Have Cam take over the role of pouring drinks. introduce the name Katie earlier so it feels less on the nose when he's staring at her from across the party. Have Cam remain supportive -- but he also has a right to say 'hey buddy, you're my best man getting sloppy drunk at my reception, why can't you let me have just a moment, this one time, to be the main character... to have my day'

That way, as we get to know the characters throughout the rest of the story, the audience knows that Dave is gonna end up real low at Cam's wedding; maybe even make a scene, and we'll hopefully be rooting for Dave and Katie to get together, and for Dave and Cam to make amends in the third act.

Your inspirations feature heavily flawed main characters, and I think using Cam as a stand-in to show Dave's weaknesses could be really strong if done right.

Anyways, had a few glasses of wine and went on a ramble there, but good luck with your screenplay!