When you notice something and want to talk about it, even though it's a little uncomfortable or unusual to talk about, you can use this phrase:

I couldn't help but notice that you're not wearing your wedding ring. Is everything OK?

A common use case is when you want to start up a conversation with a stranger:

Excuse me, miss. I couldn't help but notice the book you're reading. What do you think of it so far?

You can also say "I couldn't help noticing ___":

Hey, I couldn't help noticing the tattoo on your inner arm. It's really cool. Does it mean anything?

Aside from "notice", you can also say:

• I couldn't help but think (something)
• I couldn't help but overhear (something)