Behavior always has a Positive Intention.
This is one of those blindingly obvious things that just eludes as all from time to time. Basically, the point here is that anything someone is doing is because they are trying to achieve some goal for themselves. From their perspective, there is a reason and a motivation for their behaviors. Even in cases where those reasons or motivations seem irrational to us, or are hidden from them.
Our whole intellect is designed to pursue our desires and achieve our goals. Even in cases where we aren't aware of what those desires might be. For example, we see this in our preservation instinct and our self-defense mechanisms. Our brain resolves all the inputs and formulates responses that will further our internal goals. This is why some people are spenders and some are savers, some people are aggressive and some are timid, and so forth. Internally, they have goals in mind which motivate and drive their behaviors.
When you understand that people are never just acting in a vacuum, and are always acting in alignment with their goals, it becomes easier to empathize, understand, collaborate with, or even control them. When you are aware of their goals and motivations, you can predict or rationalize their behaviors. When you witness their behaviors, you can derive their goals.
To take this a little further, consider people watching a sporting event or chess match. It's easy to assume each player or coach just wants "to win". But the reality is that they have other goals which dictate the constraints and subordinate goals to winning. For example, they might want to manage exposure to risk, protect certain players or pieces, or have a preference for certain techniques or plays. These constraints and subordinate goals will impact their decisions and behaviors. So when people ask "Why'd he call that play?" or "Why did she try that attack?" they are only verbalizing that they don't understand these other non-obvious motivations.
The twist on positive intention is due to the nature of perspective. Often I would substitute the word Purpose instead of Positive Intention. This is because most people understand positive to "good" or "beneficial". In reality, the only person that is true for is the one demonstrating the behavior. It might very well be painful or hurtful or "bad" to others. But in their mind it's serving a purpose. Perhaps hidden and subconscious, but very real.
To put this in practice, pay attention to how people behave when the goals are very public. You will still see them act in unique ways which gives you clues to their hidden goals and motivations. For example, watching people shop is a great way to get a view into their psyche. Do they check prices first or follow colors? Do they check sizes before saying they like something? These are simple examples but they can be extrapolated to how people order food in a group, the questions they ask about the news, or how they act at a party. Are they thrifty, self-conscious about their weight, a leader or follower? The key to unlocking most peoples inner picture of themselves starts with simple observations like this.
In every situation, we are individually running all the inputs through our internal goal-seeker and deciding on a response that best gets us what we want. Watch what people do when the goals are obvious and you'll find out all those other hidden goals they don't even know about themselves.