こいつ、くすぐると笑うんだ。の英語

He laughs when he's tickled.

go [be thrown] into gales of laughter

You are always smiling, aren't you?

This is laughable.

You will be laughed at.

to laugh mockingly

I get absorbed in your smile.

Why are you laughing? I'm trying to be serious here...

You will only get laughed at for your pains―（他動詞構文にすれば）―Such conduct will expose you to ridicule.

You will only get laughed at for your pains―（他動詞構文にすれば）―Your conduct will expose you to ridicule.

I like your smile.

You will only get laughed at for your pains―You will be a fool for your pains―（他動詞構文にすれば）―Your conduct will expose you to ridicule.

an obsequious smile

You will surely end up smiling.

It will no doubt make you smile.

You'll probably laugh.

I bet you'll laugh.

You are always smiling.

Keep on smiling.

a compelling smile

burst into laughter [tears]

When are you going to stop laughing like an idiot?

give a hearty laugh

manage a smile

Please keep smiling forever.

You have a beautiful smile.

I smile when I am looking at that.

Your laugh makes it fun for me.

I am happy whenever you smile.

So, please laugh.