I also use them in birthday threads. Those don't tend to bother my eyes. It is the long paragraphs ... I can read a little bit, but then the words all seem to start running together and I simply find I cannot focus on them...and I just give up.
Lately I have stopped trying to read them altogether.
I am sure it is my age...and my eyes....I am just relieved that it is happening to others. I thought it might be something really weird with my eyes...seriously.