What kind of a person would deliberately sabotage another person's attempt to quit a bad habit?

I quit smoking for about 8 years. Had a couple of babies. Then got divorced. Then started dating this guy who smoked. About six months after we started dating I picked up a cigarette. NOT his fault. He wasn’t even around when I did it. I just….started smoking again. :(
About 3 months later I decided to quit again. I knew what to expect and I was prepared for it. I got through a week and was well on my way.
Then one night my son, who was about 6, hurt himself and I took him in to the ER for stitches.
I called my boyfriend, who I only saw on the weekends, to tell him. I was just looking for someone to talk to for a sec. It wasn’t a life threatening injury but it still leaves you kind of shaky.
To my surprise he showed up at the ER. He sat next to me. He offered me a cigarette. I said, “No.”
He said, “Go on. You need it.”
I said, “No!”
He kind of waved it in front of me and said, “It’s OK. I’ll watch the kids while you step outside.”
I wish to God I’d stood firm but…I took it. I’ve been smoking ever since.

Why did he do that?

I am not blaming him. It was my own fault. I know it was. The question is, “Why would someone do something like that?”