missing is really sweet when u have no desire to see this guy again ( may be ex-gf/bf or u know u cant not be with him/her in ur rest of your life , but u do concern about him/her very much ) or u know u will meet him/her sometimes in the future , and it will push u to some status and u can do everything positively and everything seems alright.
but when u dont know what will happen tomorrow and this relationship has a lot of uncertainty ... and u want to see this guy , keep in touch with this guy but you cant , this kind of missing is quite painful in my opinion.
anyway , i am just discribing "missing", not explaining what is missing
i guess ,missing is , u always think about a guy all the time , all the time , all the time.