Dreams about ex boyfriend and his gf but I have a boyfriend?

So I haven’t been with my ex for 2 years and I was completely for him and he was my first pretty much everything and when we broke up I was in a bad way for a long time and it took me a while to get over him. I have now been with my boyfriend for nearly 8 months and we are happy. I don’t have my ex on any social media we don’t speak and haven’t for a long time.

Last night I had a dream that he cheated on his current girlfriend with me, which when I woke up took me by a big surprise that I was dreaming that and that he was in my dreams after so long because this hasn’t happened before.

Is there a reason why I am dreaming this when I am happy and content in my current relationship and don’t ever think about my ex?