To me, this means that an exciting life is not particularly stable. To think about this, think about the difference between the feeling of falling in love with someone and the feeling you have when you have been married to that person for 20 years.
A lot of people talk about marriage and long-lasting relationships like that as if they are boring. They talk about how there is no thrill in such relationships. And they are right to some extent. There is a big difference between the thrill of falling in love and the settled stability of a long-term relationship. You might say the second is boring if you are so inclined.
When you are first in love, though, a lot of the rush is the uncertainty. You don't know whether the other person will return your love. If they do love you back, you don't know how long it will last or how it will turn out. You have no idea what will happen next. It's exciting, but in a way it's a lot scarier than life is when you have been together a long time.
So security and boredom go somewhat together. When you feel secure with someone, you're not always worried about what will happen next, but you're also not excited about what will happen next because you feel way more as if you know what will happen (that's stability).
People often confuse contentment or peaceful feelings with boredom. Insecurity, or not feeling safe, can feel exciting for many people. So, in the absence of drama/danger, boredom feelings can arise (flourish).