Why do relationships sour?

Answer 1

uhh... cheating haha ...thats mostly it..i guess

Well bad or no communication, mistrust, financial problems if they are married or living together, less sex or no sex at all, promiscuity behaviour or difference in characteristics, difference in likes and dislikes and the list goes on.

Answer 2

because people change their minds, they find someone else more attractive or compatible. there are lots of reasons, maybe you should ask someone who cares instead of the internet.