When it comes to our expectations from others we often tend to place a burden on them that sometimes far exceeds what we place on ourselves, or their capacity to handle. Be it personal relationships or political expectations, we sometimes tend to be overreaching in our expectations in a way that ultimately leaves us with a lot to be desired from the outcome.
Being measured is not a flaw. It is neither a vote of no confidence in the third person nor a slight to our decision making capabilities in terms of choices, It is however an acknowledgement to the very limitations that have prevented us from fulfilling all of our desires, dreams and aspirations as humans.
When we end up disappointed we either take it out on the other person or tend to further insulate ourselves from others and expectations by being over reliant on self. By then it might be late as the disappointment tends to propel and motivate us in ways that may not necessarily be good for us. What's more, there is the feeling of 'time wasted' from putting so much effort in the belief that the other person will substantiate our desires.
I think we do ourselves a big favour by being measured and realistic. We sometimes disappoint ourselves so why should it be so surprising that another person will do likewise. Also when we place too much we border on the unrealistic, which makes disappointment rather certain.