You know, the kind of person who you tell stuff like "I really want to go to this thing" and they point out everything wrong with it.
Such a person might be a naysayer, "a person who habitually expresses negative or pessimistic views" (TFD)
A wet blanket:
a person who spoils other people's fun by failing to join in with or by disapproving of their activities.
a person who says something will not work or is not possible : a person who denies, refuses, or opposes something
There are always naysayers who say it can't be done.
Those naysayers who attack the integrity of the entire judicial system
Lots of possibles depending on context - party-pooper, grinch, killjoy, spoilsport, grouch, wet-blanket, eeyore, misery-guts ...
A Debbie Downer, taken from the name of the title character of a series of noted Saturday Night Live sketches.
A [Stick in the Mud].1 It's a person who clings to the past or the way things have always been done and therefore not willing to listen to or try new ideas.