Tossing a scene can be really, really hard. I know. I've tossed a ton of them.
With Meg's Melody, I have a deleted scenes folder that is as long as the book. Sometimes I cut scenes because they weren't adding Meg or Matt's emotional journey. Sometimes I needed to show a concept or growth or past about a character, but decided that the way I'd originally told it wasn't the best way. And some scenes, I had to cut because my publisher needed it to be a certain word count--so I looked for my weaker scenes.
Right now, I am in the final, final editing stages of my ms and there is one last scene I am tightening up. Last night I rewrote the scene, but decided that I didn't like my rewrite at all. It was an experiment that succeeded because I now know where I don't want to take that scene. So around midnight (since I'm just obsessive enough to not be able to sleep when I have a scene that needs finishing) I made a list of what I need to accomplish with my scene.
It looked like this: (picture scraggly handwriting since I wrote this in the dark before getting in bed)
1. Gabe=Tim (a replacement for the lighthearted/normal friendship she's been craving)
2. We learn more about Philip from Gabe--get inklings of Philip's past so "the past" revelation isn't so surprising
3.Give Brenna a fun, lighthearted night (she needs it)
4. Establish that Brenna and Philip are friends now--not just people that hang out because they have a connection to Brian
5. Hint at Philip's deeper feelings for Brenna (very slightly)
So, this list probably doesn't make sense without knowing who my characters are, but I wanted to give you a sense of how I edit. If a scene just isn't working for me, I make a list of what I'm trying to accomplish in that scene, and think about how to rewrite, bringing those ideas out even more--while conveying the tone I want for that section (in this case, everyone needs a break from all of the heavy stuff that's been going on in the book so far) in addition to making sure the main theme of the book is still being supported.
Then I save my original scenes in a folder called "deleted scenes" (original title, I know.) Always save those scenes. You might end up using parts of them somewhere.
How do you decide which scenes to cut and which to keep when you're editing?