That's how we are in life

"We are two single people who just happen to be in the same carriage, and that’s how we are in life, and we’d both better believe it."

I cannot understand the part "that's how we are in life" in the novel. Would you please explain it?

Source: https://books.google.com.tr/books?id=QMLjAwAAQBAJ&pg=PA119&lpg=PA119&dq=%22We+are+two+single+people+who+just+happen+to+be+in+the+same+carriage,+and+that%E2%80%99s+how+we+are+in+life,+and+we%E2%80%99d+both+better+believe+it%22&source=bl&ots=R8tLigAzBq&sig=DLf0FqFnZs0irI7rnM62jd6XbIE&hl=tr&sa=X&ved=0ahUKEwjb76eA7ZfTAhWBqxoKHZp2AT0Q6AEIGTAA#v=onepage&q=%22We%20are%20two%20single%20people%20who%20just%20happen%20to%20be%20in%20the%20same%20carriage%2C%20and%20that%E2%80%99s%20how%20we%20are%20in%20life%2C%20and%20we%E2%80%99d%20both%20better%20believe%20it%22&f=false

Thank you.