Like the rest of the comments, sadness does root from being bored or negative thoughts. You could find a way in which you could let out those negative thoughts/feelings, such as writing them down, coloring, listening to music (if you’re feeling sad, don’t listen to sad music, try upbeat, and happy songs) or even talking to a close friend about what is bothering you.
A teacher always told me that we are like balloons. Every time something bad happens it blows our balloon and the more and more bad experiences/thoughts/feelings we have our balloon can only contain so much that we will reach our tipping point and blow up (just like a balloon when it’s filled too much). This is why we should channel those negative thoughts and let out that air so our balloon is nice and small.