A major stepping stone in the topic of Hashgacha Pratis is Ramban on the following posuk:
For I have known him, to the end that he may command his children and his household after him…
Ramban, commenting on this posuk, wants to know what the Torah means when it says that God “knew” Avraham:
Paraphrase: It is true in my eyes that this was a real knowledge. God’s supervision in this world is generally to simply preserve the principles. Man, too, depends on the random realities of nature until his time comes…
In general, then, God maintains and perpetuates nature. He makes sure that the sun rises and sets, that the moon waxes and wanes, and so on and so forth.
Thus: When something happens to a person it doesn't have to be reward or punishment from God. The person just got affected by the random realities of science and nature. If someone is standing beneath a decaying wall, and it falls, this does not mean that God pushed the wall at that moment. Rather, the elements of the wall simply eroded over time, according to the laws of science, and it collapsed. This person just happened to have been there when it fell. In short: Don’t blame God; stuff happens.
Paraphrase: However, when it comes to God’s pious people, for these people God gives special attention. He wants to know them privately and specifically, and protects them and is connected to them always.
Regular people are not privy to this level of Hashgacha Pratis; this higher level of a detailed Divine supervision. Only the chassidim/pious people merit this level of Divine supervision.
There is, of course, reward and punishment for everyone, but random occurrences can affect everyone as well. As in, while some things that occur to a person could very well be Divine retribution or reward, not everything that happens to a person must be viewed through this lens. Sometimes, it’s just coincidence(/nature/science/pick your own term here).
For the chasid, though, nothing will happen to him by chance. There will be reward and there will be punishment, surely, but God will protect him from chance occurrences and nature. Anything that happens to the chasid, we conclude, God wanted it to be so.
Paraphrase: As it says in Job, “God does not remove his eyes from the tzadik”, and there are many other similar posukim to this effect.
The word tzadik and the word chasid here are interchangeable. The Divine protective umbrella, if you will, is relative to the piety of the person beneath it. Not everyone merits such a level of Divine protection. As we will discuss, one’s level of Divine protection is commensurate with one’s knowledge of God. The more one “knows” God, the more God “knows” the person. There is obviously a minimum standard required for this protection, but even once you’re in the club, so to speak, not everyone is equal. In short, and as we shall see at length, the concept of Hashgacha Pratis/Divine supervision is very much reciprocal.