Hey, it's not like you have to be all sunshine and rainbows. You're a professional working with other professionals, and it's business not kindergarten. But while not every idea is a winner, you don't want to be the person who becomes notorious for automatically dumping on every single idea in a brainstorming session or planning meeting.
If you're so judgmental and critical of new ideas, eventually people will feel less and less comfortable sharing things with you or in front of you. At that point, whatever new ideas and valid criticism you bring to the table might go ignored because everyone is used to your negativity and no one wants to hear how stupid they are time and time again.
A little constructive criticism -- and some tact -- goes a long way.