Historically, “lo!”, isn’t expressive of any particular emotion (alas) or addressed to any particular person (dude), and it's not an all-purpose interjection (Hey). It expressly calls upon hearers to look at, to take account of, to behold what follows.
In contemporary English we say “look!” in pretty much exactly the same way.
He claims he is innocent of corruption, but look! He is guilty of taking bribes!
For that matter, so did Shakespeare. Hamlet, I, i, 40, the Ghost’s first appearance:
Marcellus: Peace, break thee off; look, where it comes again!
And at the second appearance, 86 lines later
Horatio: But soft, behold! lo, where it comes again!