Impermanence, also called Anicca, Aanicca, Anitcha or Anitya, is one of the essential doctrines and a part of three marks of existence in Buddhism. The doctrine asserts that all of conditioned existence, without exception, is "transient, evanescent, inconstant". All temporal things, whether material or mental, are compounded objects in a continuous change of condition, subject to decline and destruction. The concept of impermanence is also found in various schools of Hinduism and Jainism.
Impermanence is intimately associated with the doctrine of anatta, according to which things have no essence, permanent self, or unchanging soul. The Buddha taught that because no physical or mental object is permanent, desires for or attachments to either causes suffering (dukkha). Understanding Anicca and Anatta are steps in the Buddhist’s spiritual progress toward enlightenment.
Impermanence and Attachment As long as there is attachment to things that are unstable, unreliable, changing and impermanent, there will be suffering – when they change, when they cease to be what we want them to be. (...) If craving is the cause of suffering, then the cessation of suffering will surely follow from 'the complete fading away and ceasing of that very craving': its abandoning, relinquishing, releasing, letting go.
— Rupert Gethin on Four Noble Truths
This is why signing a permanent contract with a woman is a bad idea. This is why you get upset when things do not work out with the relationships you are attached to.
Impermanence is neither good, nor bad, it simply is what is. Most initial reactions to this will be negative, or try to fight against the concept of impermanence. This is foolish, because even such reactions and attempts are subject to the very same impermanence in themselves!
If you can see that there is impermanence, simply enjoy your ride on the river and try to have a good time. Avoid making permanent commitments due to the fact that permanence does not exist.