Its not hard to understand at all, if we realize its all about the four various states of matter. There is the plasma state, the gas state, the liquid state, and the solid state. And if you want quibble, we can add nuclear transformations too. But if we want to restrict the question to normal earth conditions that make life as we know it possible, we can somewhat eliminate the plasma and nuclear questions.
Depending on the chemistry of the atomic and molecular states mixtures in question, we can get wildly different results as we vary temperature and pressure. ( and its hard to talk about H20 here because its a rather nonstandard behaving non-conventional liquid. )
But as soon as we talk only about the solid, or gas, or liquid question, we have to immediately ask, why should any given atom, stick to any other another, regardless if they all have the same number or protons in their nucleus or not. And we can only explain it with the behavior of electrons between two of the same or different atomic numbers atoms.
In a pure gas, the inter atomic attraction is zero, although we have many gases that bond together and gain a tiny bit of energy as say two nitrogen atoms stick together to become N2, or two Oxygen atoms become O2. They still act like a gas that will get ever further apart in a vacuum, or can be packed ever tighter by adding pressure.
But the liquid state of matter is quite a different State, suddenly its gains lots of energy
as it packs to minimum volume and can't become smaller with the addition of pressure. But still liquids can assume any shape, and can be poured from one container to another of different shape. Only volume must be conserved.
The we can talk about the solid state where even more energy is gained as it preserves both of its volume and its shape. And suddenly we can ask how hard, soft or brittle it is? At least until recently, the conventional solid state thinking was confined to saying, there are only a limited number of ways metals could crystallize in, known 3000 years ago by the Greeks, and now the recent Nobel in Chemistry has been given to an out of the box thinker who has proved not even that is always true.
But still getting away from the liquid to gas boiling point question. Because the key to understanding that is in statistics and molecular. Why should we assume that all atoms in a mixture are going at the same velocity even if the overall mixture is stable in temperature, especially in a liquid. Because all the atoms are always banging against each other, many many times every second. Some slow to almost zero in a collision and have a temporary temp of absolute zero, and other speed up and have a far higher temperature. In other words, for any given atom, temporary velocity equals temperature. Which means at any given time every atom might have sufficient velocity to tear apart its attraction to its neighbor it was stuck to before. Now if we apply that to a pot of water at some altitude being heated from below, we can finally understand what is happening. Because those temporary higher velocity water molecules at the air liquid interface have a far higher probability of escaping the water in the pot, and becoming just a gas in our atmosphere. Its a meta stable reaction rate, how many water molecules that escape the mixture compared to how many slow down and join back up with a another similar water molecule before escaping the mixture.
And that also explains why all those free H2O water molecules in our atmosphere later join up and fall as rain, and or snow when temperature and pressure change.