Why did Fiona have to cheat on Mike?

I'm re-watching Shameless for the second time now, and it makes me so mad that she had to cheat on him. I can't even describe it, she was moving up in her career in a corporate, office job. She was dating a pretty cool dude, he was hot, great smile, smart, loyal, good at his job, and she had to fawn over his annoying bad boy trope brother??

Does Fiona have a thing for bad boys? Is she an adrenaline junkie?

After that her whole life started falling apart, Liam and the coke, she got fired of course, Mike found out about the cheating.

I know Fiona's destructive, chaotic even... everything was going so well for her why'd she have to ruin it for god's sake??

I thought Mike was her best lover/boyfriend. He had a stable job unlike Jimmy/Steve who was a car thief and couldn't keep out of trouble. He was stable and didn't seem to have any problems? Addictions, unlike Sean. He wasn't married, or lied to her ever. Aka Ford. Gus was alright too, but I preferred Mike as a SO to Fiona. I love Fiona, but after she cheated on Mike my "like" went downhill.