对你微笑纯属是因礼貌看你面带微笑非奸即盗
[duì nĭ wēi xiào chún shŭ shì yīn lĭ mào kàn nĭ miàn dài wēi xiào fēi jiān jí dào]
A more elaborate phrase that says 'my smile is purely out of politeness, but your smile looks insincere and maybe even suspicious' - a mix of social obligation and mistrust.