Why did my bf say these things to another woman?

I caught my bf through his snapchat talking to an old female friend which he & she flirted and kissed back then, nothing more. I was so mad because he’s my best friend and he lied straight to my face when I confronted him about it until I be admitted that it was flirting. So on his snapchat the convo within this female was as:

My bf: “You are amazing and super cute”
Female: sure
My bf: “I’m serious i still Have a big crush on you..”
Female: please stop
My bf: I know
Female: Ima just go to sleep now
My bf: sorry...sleep good. I’m ******* stupid. Sorry.

The female then doesn’t respond. The very next day my bf comments on her snapchat picture cutie.

Few days after...

My bf: if everything goes planned I’ll be going to Cali in October.
Female: that’s cold for how long?
My bf: A week hopefully. Maybe more. If i do Go over ..want to hang out?
Female: maybe
My bf: I honestly miss talking to you and hanging out with you. I miss You.
Female: it happens..
My bf: do you miss me?

He swears up and down that he wouldn’t have actually hanged out with her that it was all talk because we were going to fly to Cali in October. To visit his family. I ask Him over and over why did he flirt with her and how long and he said it’s been months on & off but he really doesn’t know why he was flirting and saying those things. I would Like to know why he can’t give me an answer because i find This bs. But it may be a different answer than mine if a males perspective is explained.