This is going to sound quite pessimistic and cynical but please read on if you have the patience.
When dealing with fellow human beings, expect disappointment, unpredictability, and irrationality. Why? For several reasons. Keep these observations in mind:
1) When dealing with an individual in a "cold" and emotionally unaroused state, don't assume they are going to be the same way when they are under the influence of emotions of any kind, whether it be happiness, anger, sadness, stress, confusion, etc etc.. When we are unaroused, we assume that we will act in a logical and consistent manner while under emotional arousal or stress. Often times, this is false and we have absolutely no idea how we will act or what we will do when we aren't in a cold rational state.
2) We come from different backgrounds and drastically different experiences. This can lead different people to view the same situation in radically different ways. We really have no idea why we hold the preferences that we do and why some things that frustrate us greatly seem to have no emotional effect on others. There is no way to accurately identify the root cause for a majority of our predispositions.
3) Neurologically speaking, we do NOT know how decisions are actually made and what happens in the brain, at a molecular level, when an individual has to make a choice. It's still a mystery. So, when a person makes a decision, at the most basic level, we have no idea how that decision was made and what kind of biological influences and reactions were at play. Why did you decide to cut that driver off today but restrained yourself yesterday?
4) We are unconsciously influenced by a plethora of contextual and environmental factors. The sheer amount of ways that we can be emotionally/mentally primed without even realizing it is immense. The power of roles, costumes, anonymity, authority, and an unpredictable/unfamiliar environment can have a powerful influence on who you think you are and how you think you will behave in different contexts. Consider yourself living in a first-world country in an urban setting and then realizing how differently you might act if you were placed in a third-world rural village with contaminated food and water and no social support of any kind for miles. Chances are that you will behave in ways that will surprise you. Does this mean you aren't who you thought you were?
5) Our willpower and ability to practice self-restraint varies throughout the day and even depends on how well fed we are. An individual who has been dealing with crying children all day is going to be much more on edge and "touchy" than someone who has been lounging at the beach. Does this mean that the first individual has less self-control and is just a more frustrated person in general? Absolutely not, that person's willpower has simply been depleted and their emotional restraint has diminished.
This list isn't exhaustive and there are many more factors at play that can lead to an individual behaving in unpredictable and irrational ways. Given these observations, is it worth actively seeking and fostering friendships? Is it worth falling into perhaps the greatest emotional abyss of them all? I am referring to love and relationships of course. The answer is YES to all of these situations. But, before you set yourself up for failure and disappointment, realize that you are dealing with a creature that is operating under all of the above restraints (and then some). Don't create high expectations and be more forgiving when shit (inevitably) hits the fan. We're only human.