The natural flow of life alternates between happiness and sadness, excitement and dullness, moments of creativity and several moments of the opposite, contentment and dissatisfaction, and yet it is possible to experience this with loving attention. This is what meditation is about. Witnessing the experiences with love and kindness.
Being able to witness the vast range of experiences from a kind/loving standpoint is what makes one realize that they are not their thoughts, they are not their emotions, they are not their physical sensations (body). They can witness all this, just like they can witness the sunrise or their immediate surrounding, including other life.
So why then do we have such range of experiences? We go up and down. There is no absolute answer to this question. It can depend on how you want to answer it. Just like why is there water? It just is. You could break it apart and say water is made of molecules, and smaller constituents, and try to explain the properties. At each level, you can still find something deeper. How you want to see it, changes what you see.
From one perspective, what you call as ‘you’ goes up and down for no reason. It just does. Just like the wind blows. Or just like you add meaning to experiences. But looking from another perspective which we as humans would appreciate more, there is an order, direction to things. While in the grand scheme of things, this order or direction is inconsequential, but each constituent has some order/direction. Now that if you have an order/direction, naturally you will seem to go up and down, side and side, and all possible ways. ‘You’ cannot exist separately from the rest of this moment, including other human beings, other forms of life, wind, sun, or the cells inside the body.
What is this order/direction that we so appreciate as human beings. It is the natural flow of things within the whole. Now from a human perspective, going up and down, seems to create a story, that we can relate to, and that adds resonance to our experience, if we can witness it with loving attention. Regardless of the cause and effect, going up and down is like the notes of music. You cannot create good music with a monotone. That is the music of the rocks. The music of humans and living things has more complex notes. Notes are synchronous and yet different parts of the sequence have contrasts. You cannot appreciate a particular note in the absence of a contrasting note in another part of the sequence.
These ups and downs are in some ways helping you grow (in one perspective). In another perspective, they are helping you feel resonance in the different notes of the sequence.