cope with deadlock

[王科选]

hi,
     From this url: 
http://dev.mysql.com/doc/refman/5.5/en/innodb-deadlocks.html , mysql 
says "If you are using locking reads (|SELECT ... FOR UPDATE| 
<http://dev.mysql.com/doc/refman/5.5/en/select.html>or|SELECT ... LOCK 
IN SHARE MODE|), try using a lower isolation level such as|READ 
COMMITTED| 
<http://dev.mysql.com/doc/refman/5.5/en/set-transaction.html#isolevel_read-committed>."
     What's the reason? I have read some online material, but still 
don't get the point, can anyone explain why?