If we do CBT or practise Buddhist techniques, we will be invited at some point to notice whether experiences are pleasant, unpleasant or somewhere in between. After a while we might notice that this sense of "how pleasant something is" is a continuously shifting field of experience - it is like a dancing candle inside of us, connecting our mind to more visceral sensations. It's even there when we are not conscious of it, driving us to do things we might not otherwise do. It underpins many of our impulses, desires, behaviours and actions. In short, it controls how we behave.
You might think you know why want something, but even that is not as clear cut as you might think. Perhaps we reach for a biscuit because we feel like something nice. Quite often, we already feel quite good but we don't know why so we try to attach some real world event to the feeling - so I now feel good because I ate a biscuit, not because I was feeling OK. Indeed, if we sit and do nothing we fall into a slightly pleasant state most of the time - we are set to a default of slightly pleasant. We do not always need a reason to feel happy.
The reverse side of the coin is unpleasantness. If we experience something as unpleasant then we try to get rid of it. We might try different strategies such as replacing it with something pleasant (a biscuit) or by generating some angry energy of some sort. On many occassions we experience unpleasantness as a "lack of something" and we respond to this by giving ourselves something like, say, a biscuit. We want something but we don't know what.
We could go on and on analysing pleasantness, but it is best to explore it for yourself. We tune into it - pleasant, pleasant, unpleasant, wanting pleasant - reaching for the biscuits. You might find it a bit unusual to reduce yourself to three things - pleasant, unpleasant, and somewhere inbetween - but once you tune into it, it is surprising how much of what we do emanates from these simple beginnings.
The good news is that it's easy to feel good. We just need to let things go and pleasantness arises by itself. And we leave it there, a simmering thrum of contentment. Nothing makes it happen, it's just the way we are built.