TDHT brought up a good point. Having a pre-disposed idea or concept about how something or someone will turn out (behave) can be negative in that what you look for is what you'll get. This attitude rules out experiencing something completely new or spontaneous. Many call this "clinging to an outcome". If you have total control over the potential outcome - then the goal/ideal makes sense. If not - well - you could be setting yourself up for some suffering...
Having ideals to strive for: You - me - groups - communities - the world....these are worth dwelling upon, codifying, desirous, when it comes to bettering the greater good.
The problem comes in when we have an ideal - or expectation - of a certain result - and the end result does not match our picture we have in our mind - and then we cling to (or strongly hold on to) the fact the reality does not match our expectation.
For example: I walk into a shoe store and I expect a sales person to (eventually) show up and help me locate a pair of shoes. When no one does, I get frustrated - perhaps angry. At this point there are a few directions I can take with my thought/emotion pattern going on in my head. I can continue to get angrier, stomp around the shoe store glaring at other people, muttering about how awful this place is, bump into a small child, knock them down, etc.
Or - I can get depressed and declare in my mind: "This always happens to me, I don't know why I bother to shop at all, there's no reason for me to be here, there's no reason for me to be....".
Can you see how I'm clinging or holding on to my failed expectation? Can you see how it is causing me mental suffering that I'm creating in my own mind?
Now, once I've realized I'm making myself miserable, I can let go of that emotion/thought pattern, take a deep breath, and proceed to the "service desk"; calmly ask them if there is someone who can help me. Or I can tell them I'm quite disappointed in this store and will never come back. Either way, I've kept my ideal - my expectation of what I consider good shoe store shopping experience and I'm not suffering any more.
It's the holding on or clinging to one's expectations that cause the problems. NOT the expectations.
Another example:
I thought my husband, my mate, my partner, should help me out in all of my endevours. The role of spouse is well defined in my head and I expected him to live up to the role. When he did not, due to his own limitations, I eventually became very angry and kept holding on to the idea that he should change himself - or get some therapy - or Something - in order to fulfill my expectation of the role of my spouse. I made myself very miserable by holding on to that ideal. I made him miserable too, for a long time. When I look back on this time, I wish I had been more aware of my clinging. Because as soon as I let go of holding onto the illusion, my life became much easier as I began to accept the reality. I was able to make better decisions for me. I was able to take care of me.
Having ideals, expectations, and goals in our life give us meaning, excitement, and I like to think, a better world to live in. Sure, there is a fine line to walk because one has to figure out just how much they can control within their expectations. But clinging and holding on only delays the inevitable, the disappointment when reality does not match up to our fantasy. Better to let go, and see what really is, so as to find solutions to why it didn't meet up.