I look forward to seeing you.为什么see加ing

sb look forward to doing sth是固定用法,动词用进行时,不用一般时.

从语言习惯上来将,当你look forward的时候,看见对方,肯定是正在看,所以选择用进行时,从语法上将,这里的to是介词,介词后面的动词应使用进行时态

是固定用法!
给你找了英语的解释!
Look forward to is one of the many phrasal verbs in English in which an adverbial particle (forward) as well as a preposition (to) is combined with the stem verb to signify a particular meaning.What we are looking forward to can be exemplified as either as a noun phrase or as a verb-phrase with an -ing pattern

look forward to 中的to 是介词,所以要加动名词（doing).