Why Do Matter-Antimatter Pairs Annihilate Instead of Sticking Together?

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Why does an matter-antimatter pair annihilate each other instead of "sticking" together (i.e. like ordinary matter with opposite charges do)?
 
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In fact, they don't always annihilate right away. For example, an electron and a positron may form a bound state called positronium. Positronium doesn't live forever, but a certain form of it can live for a relatively long amount of time, more than 100 nanoseconds! That may not sound like much, but it does show that matter and anti matter don't have to immediately annihilate.

http://prl.aps.org/abstract/PRL/v90/i20/e203402
http://physicsworld.com/cws/article/news/17559
http://en.wikipedia.org/wiki/Positronium

Hope this helps.
 
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A relativistic positron can annihilate in flight with an atomic electron in matter. The integrated probability for a 50-MeV stopping positron is ~ 15%. See pages 384-6 in Heitler "The Quantum Theory of Radiation" Fourth Edition.
Bob S
 
Thank you very much for the feedbacks! I appreciate it. =)
 

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