sininle  means with 

& why hayal etsem not etim 

It is a condition. He always cries when he imagines to be able to laugh with her. So to express that condition we use -se.

thanks alot gokuyum but if there is a link to post that explain this se and the meyi part can you post it please 

I am not good at finding links but I am sure abla can help you.

never mind I will wait for her help thanks again

About ise or its suffixed form -sA- so much has been written on this site. With this small piece of language if clauses are formed in Turkish. -sA- denotes a real or hypothetical condition on what is expressed in the main clause: hayal et|se|m ´if I imagined´. Read for instance how Henry has explained it here:

                        http://www.turkishclass.com/forumTitle_52547

In your sentence

                        Ne zaman seninle gülebilmeyi hayal etsem ağlıyorum

pay attention to the verb hayal etmek ´imagine´. The linked dictionary says

                         "hayal etmek /ı/ to imagine"

 ı between the backslashes means this verb governs an accusative object. You can´t make accusative of a verb, can you? That´s why -mA- is used in the middle to change the verb into a noun. Thus, once again

gül|ebil|me|y|i = ´to laugh´ + ´to be able´ + VERB > NOUN + BUFFER + ACC

Small addition; check this link for "whatever/whoever" etc.

http://www.turkishclass.com/forumTitle_53864