What's the true meaning of happiness?

This is such a great question, and as such, can produce a plethora of different responses! As a means to provide an appropriate response, it's important that we take some time to break down aspects of desire and pleasure, as well as human nature surrounding this and mindsets. In essence, we as humans are wired to seek pleasure and avoid pain.

When we experience continuous instances in our life of which don't produce a sense of joy, it wires in our brains all or nothing thought patterns. For instance, if someone were to continually be ghosted despite numerous inquiries for social engagement, it reinforces the thought pattern that "no one wants to spend time with me." In essence, themes of lacking happiness (which we call anhedonia in therapy) can be a plethora of conditioned responses based on the example I provided, as well as genetic components of which one may be less prone/sensitive to the experience of happiness as a whole. Even though it may not seem like it, anhedonia and depression are defense mechanisms.

The mind is predictive, and bases such predictions on past experience. It's important to note that what we focus on, grows. If we continue to harbor thoughts in our mind such as, "I will never experience happiness", or, "It's only a matter of time before this person ghosts me", our thoughts will take hold of our emotions, and in turn, root in dissatisfaction and anhedonia. However, if we do the work to identify such thinking patterns that don't work to help us, we can challenge them in healthy ways with a skill called "re-framing." Re-framing is where we identify thoughts that don't serve us, and frame them in a way that begin to serve us. For example, the thought, "No one wants to hang out with me, I am better off alone", can be re-framed to, "Although there have been instances where I have been ghosted, I am hopeful that the right friend comes about for me." Call it wishful thinking, but the more we focus on the latter thought, the more likely it is that happiness will begin to come to fruition more often in our lives. 

We can also achieve more periods of happiness by leaning into our values: the things that are important to us. When we begin to gain insight as to what we have passion for, and in turn, create space for such passion, we find that happiness isn't a state of constance, rather, something that comes and goes. When we cling to it too intensely, we actually experience less of it. 

For what it's worth, a combination of mindfulness (the observation of the present moment, without assigning meaning and judgment to it) and re-framing techniques will likely aid in eventual days of experiencing more happiness.