you had to see
The phrase you had to see (or hear) is used to tell someone about something unbelievable that they missed. In this usage, it means the same as you should've seen. This can be something positive, like You had to see the look on Jim's face when his wife told him she was pregnant. Or something negative, as in the following snippet:
Nicky: So, last night, Natasha and I went to dinner at that new Thai place downtown, and who should be sitting at the next table but your ex boss and her husband.
Ronnie: What? Did they see you?
Nicky: I don't think they know who I am. Anyway, they're whisper-yelling at each other when suddenly he whips out a document and slaps it down in front of her and says, "I want a divorce!" You had to see the look on her face. I thought she was going to reach across the table and strangle him.
Adam tells his coworker, Dennis, about the company picnic the day before.
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Adam: Too bad you couldn't make it to the company picnic yesterday. I actually had a blast. But you had to see Carl in Accounting... holy shit!
Dennis: You mean that nerd in the corner desk?
Adam: Nerd?! He took off his glasses and his shirt and he's like totally ripped. He downed half a bottle of whiskey and still won the arm wrestling competition. Then he got up on stage and won the karaoke contest!