As many brands of philosophy teach, letting go of (not having any) expectations of the outcome of any given situation allows us to live in the moment. It allows us to be content and happy at all times. It allows us to be present.
If i go in to my favorite taqueria expecting to have my favorite burrito just as i have had it a dozen times before, and this time the guacamole tastes different and the salsa is a bit hotter, i am on a path to disappointment. My expectation is the crucial factor here.
If i go in to my favorite taqueria with no expectations other than having a pleasant meal, it wont matter if the burrito is a bit different than usual. My expectation is tht of optimism of the experience rather than an expectation of the experience.
This is even more true with relationships. If we expect our friends or spouses to behave a certain way, we are setting ourself up for a let down. Each person is an individual. It is human to have expectations of others, but it is detrimental to emotionally invest ourselves in those expectations. Every human is on their own path. Our spouses and friends have no obligation to be who we expect them to be. They have an obligation to follow their own heart. Each person owes that to themself.
Rather than have expectations of others behavior, it serves us all to just love them for who they are. To love them unconditionally as individuals. No expectations, no let downs. Adore who people are rather than note who they are not.
It is important to note that desire is not the same as expectations. Desire is very heathy. But it is best to apply our desire with broad strokes. Here is an example. If we are shopping for a home and we want a home that is blue, two story, with shutters and a water fountain in the front yard, we are destined to be let down. If we are shopping for a home and our only expectation/desire is that it is a beautiful comfortable home that makes us happy and that we can afford, the options become endless. Painting desire with broad strokes not only minimizes our potential for disappointment, but opens up a world of options. We may find things we had no idea that we even wanted until we saw it. We may end up with a single story Mediterranean home that we adore far more than the blue home we thought we desired.
Letting go of expectations is liberating. If you have no expectations, you can never be disappointed. No expectations, no stress. Things will rarely unfold the way that we expect them to. Rolling with this rather than resisting this is key to happiness.