I totally identify with not being able to read my own writing sometimes! For me, I think, mood is a much more significant factor when I'm trying to critique my own work just because I'm naturally so much closer to that piece of work than I would be to anything anyone else has written, and I often find myself forgetting that if you've written and re-written the same piece several times over, of course it's going to read as predictable and overworked because it's yourself writing it and working on it! That can get me into a really negative spiral sometimes. When it comes to critiquing the work of others though, I'm much more critical of why I feel the way I do about a given piece of work. If I can't find pieces of evidence that I can specifically highlight to support my initial feelings about a piece, I stop writing my critique, re-read, and try and assess the work on what is actually present rather than just on my mood-dependent first impression. I think when it comes to giving in depth feedback, though, this happens naturally as you try and put together concrete improvements that people can make. I think also when I'm critiquing myself it can be easy to indulge the desire to rubbish my own work and tell myself I'm terrible, whereas when it comes to others my focus is always much more on helping.
When you are reading something normally then you just run through it and as long as the things fit together and don't jump out at you as being bad then they don't matter. You focus on the big stuff, the whole of the work. But when you slow down you are, in so many words, looking for 'mistakes'. It's the difference between looking at a painting from three feet away, where you can see the whole thing and appreciate the complete work; and looking at it from two inches away where every tiny brush stroke is stands out to you in a way that they don't to someone looking at the actual picture. Just by zooming in you find things to critique, and critique in this case pretty much just means criticize. You start taking things out of the greater context and breaking it down to the point where you lose sight of the whole piece. That's how you end up looking at a line of dialogue like "We'll cross that bridge when we come to it." and writing four paragraphs of critique about how that's such a cliche and a pedestrian one at that, that it lacks elegance and come on you can write better than that. It's so easy to forget the virtue of things that just do their job; to expect every phrase to weep poetry when most readers won't even notice that line; they passed over it in two seconds and are on to the next thing. That's why it's so important to make an effort to get some distance from the text and to try and come in fresh and relaxed and give the writer a break, especially if it's you. You can always find things to dislike, to set impossible standards and make out to yourself that you're just being a forthright critic rather than being a jerk because you had a bad day. I think you have the right attitude though; coming in thinking 'how can I help this person?' and making yourself think in terms of finding things that make it better, not harping on the things that make it bad. As for critiquing your own work; I put a long break between writing and editing a book. I write a book, go back and edit the previous one, write the next one, then come back and edit the first one. It's not a perfect way to work of course, but it does mean that by the time I'm sitting down to edit I've already been through a bunch of other stuff, have had another great idea and forgotten how passionate I was about this book at the time. That way I come in fresh enough to laugh at the jokes again, to feel the emotional tug and get a better idea of what's good and what's not. I know that's not how lots of people work but I strongly advise it. At least having gone and written something else and let go of what you wrote before makes you much more effective at improving your past work.
Your point about zooming in so close that you lose the overall context is a really excellent one, and also the idea that not all readers will necessarily fixate on things like small cliches or throw away sentences etc. I'm often unsure whether or not to include parts in critiques that are so specific as pointing out one particular cliche, as in your example, or whether I should be erring more towards the side of more "general" comments, especially as sometimes things that I would change in my own work are stylistic features of someone elses.
