One example could be the story of Romeo and Juliet. Their love has an arc. It starts with a sudden and intense attraction at first sight. Then they face numerous obstacles like the feud between their families. Their relationship deepens in secret, but ultimately, due to the circumstances, it ends in tragedy. So, the arc goes from attraction to deep love to a tragic end.
A modern example could be in the movie 'The Notebook'. The love story between Allie and Noah has an arc. They meet as young people and fall in love. But then they are separated by Allie's family and different social classes. Years later, they meet again, and their old feelings resurface. There are many ups and downs as they try to rekindle their relationship. The arc here shows the journey of their love from the initial passion, through separation and pain, to a final reunion.