Katy Perry and John Mayer have broken up again, and there is a reason why it didn't work the nth time around. One of music's power couples, the duo has been on-and-off ever since they got together and the relationship obviously hasn't been smooth-sailing.

The pop stars started dating back in the summer of 2012, but by fall of that year (roughly two months later), they had already broken up. Thankfully, it was a brief one, and they were on again in no time.

They broke up again for the second time in March 2013, reportedly because they have been focused on their respective careers. Mayer even told Ellen DeGeneres during an interview on her show that "Coupling is a tricky thing." Come June 2013, they were back together.

It has been their longest period of bliss since. They collaborated on the song "Who You Love" and even did a sweet video together for Mayer's Paradise Valley record. They quickly became the target of engagement and wedding rumors. They were seen together on Valentine's Day and Perry even gave everyone more to talk about when she was seen wearing a diamond at the Elle Style Awards in London.

Now the sexy singer and the brooding crooner are no more. The signs were there - Perry was alone on her most recent Europe trips and Mayer has been nowhere in sight. The reason for this latest split is simple - the couple's careers are still as crazy as ever.

Us Weekly is reporting that Perry's upcoming Prismatic world tour was the main cause of the tension in the musicians' relationship. A source says the "Dark Horse" singer has been stressed about it, resulting in fighting. Mayer's latest tour also contributed to the stress. It has been a rocky ride for the stars, and there is no telling if they will get back together again.