Why does the messaging app make multiple threads for the same people?

May 16, 2011
Whats the deal? It's been doing this for months, and seems to get a little worse with every release.

If they send a picture message it gives it a new thread, and if I sent a reply in that thread it shows up as group message. I don't know what makes it start new threads, but I have 2-4 threads for each of my most texted contacts.

This is highly annoying.