I can think of 4 good reasons why Syria would want to surrender their chemical weapons.
1. Protecting military assets costs resources. Freeing soldiers from guarding chemical stockpiles increases the number of soldiers who can fight rebels.
2. The chemical weapons are worse than useless. Any further use of them now would almost certainly bring on a military strike.
3. Getting rid of them denies them to the rebels. This is a weapon he can't use but in the hands of terrorists they would be a threat to the entire world.
4. Giving them to the UN puts the burden of guarding and destroying them on the world community. Nobody would believe him if he said he got rid of them himself.
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