He thinks life is interesting, and so __ I.

Here should I use "do" or "am"?

Or either?
(When "do" is used, that means I think the same;
When "am" is used, that means he thinks both life and I are interesting.
)

I would say: "He thinks life is interesting, and so do I." It means: "He thinks life is interesting, and I think life is interesting, too." The other sentence with am is not grammatical in standard US English. To get your second meaning, I would use the non-elliptical sentence you have in your explanation. It still sounds a bit funny yo me, and I would probably rewrite it to something else.

I agree with zmjezhd. If you want to say "...he thinks both life and I are interesting", the least awkward would be "He thinks life is interesting, and that I am, too." It's not a phrase you're likely to encounter very often, though.

I would concur with zmjezhd, also, about the pen sentence. Whenever possible, write as simply as you can. "I bought a blue pen." Think of it this way: "Me compré un bolígrafo azul." or "Me compré un bolígrafo que es del color azul." Which sounds more natural? :0)

To a Chinese speaker like Callithump, why would either sound more natural? Would either one be understandable at all?

OOOOPS!!! MY BAD!!! I had him mixed up with another... Sorry! (And thanks for the heads up, Latishya!) :0)

For some reason what comes to mind is I Am America And So Can You! by Stephen Colbert