Those who has desires can worship siva. Those who born out of prakruti will not fullfill the desires of peoples unless then compelled to fullfill or they do not have option to fullfill or for publicity they will fullfill on randam basis or you may get fullfilled also if you have luck. Except siva all born out prakruti. Siva created the sakti firstly, and then he created the Vishnu, then brahma. Vishnu and brahma are born out of prakruti and has their gunalu. Siva given the vedas to visnu. Siva teaches  the siva gnanam and how the universe exists to vishnu initially. from visnu it has speaded. Jagadguru is only siva, not humans, not visnu, not brahma, not anyone. Based on that you can understand why only siva is jagadguru.  if you ask, Why you may not be able to fullfill your desires from others, for example, there is story from siva purana, after shirasagarmadana, many beautiful girls came from spread of amrut into land(land means anything not earth, it can be in any lokha, for example, in swargam also land will be there, etc), that beautiful girls has been taken by rakshasas to rasatala lokha , and then rakshasa living good life with these beautiful girls, then visnu went to rasatala and killed rakshasa with his chakra, then started living with that beautiful girls in rasatala lokha, from that beautiful girls, many rakshasa sons of visnu came and they went and start irritating devatas and rushis. then rudra came in bull form and killed all sons of visnu. then after seeing mruta dehas of his sons , vishnu try to kill bull, then bull put with his legs vishnu to down, they vishnu prayed to rudra. then rudra told, oo visnu it is not good for you to live in sreevyamoham.  i am giving a saap those from now onwords who will take the oath king of three lokhas , they will destroyed fully if they into sree vyamoham. this story can explain everything.   Those who born out of prakruti will have punya and papa karmalu.  if you have desire, then ask desire first, then pray. and all always keep asking that desire to the god while praying. Also say that i am praying only for my desire. This principle will apply only when you are recahing god for fullfilling your desire. Above content told by siva, not by me . Source- Extracted from siva puranam. This website is under development.