Why they are clinging
               These five masses of clinging help to rebirth cycle and bond creatures in infinitive cycles of births. So they are called stick without known or clinging. Birth help to give pains, sick, getting old, death, meeting negative people, losing positive people and obstacles to achieve goals. As wholesome said this five masses are stressful. Mind is collaborations of energy packet which rapidly start, stay very little time and vanished. Till we get whole enlighten, creatures go from one life to next life as five masses of clinging. Not only mind or soul. After we get enlighten, it become five masses and not clinging. Every creatures bond to hell, hungry ghost realm, animal realm & asura realm cause of creatures think these five masses of clinging as ownself or "I" or idea of soul. It means they think there have steady soul which can control ownself. But Lord Buddha who enlighten that there have no soul & have only dependent organism (reasons & results) inside rebirth cycle.

Massive bodies (Rupa)
             In Buddhism simply physical body and it's organs called massive bodies or Rupa. Organs which related to five senses are called internal masses while subjected objects are called as external masses. As example eye is internal mass while light rays and object are related external mass. So eye-light, ear-sound, tongue-taste, nose-smells, body-feelings from skin or internal body and mind-mental visualization are called as massive bodies. Massive bodies created & maintained according to foods which creatures are eat. Basic building block are elements of earth, liquids, heat & air. 

Sensing And Identifying (Vadana, Sangna, Sanskara & Vinghagna)
            After internal & external masses get mix with consciousness (Vignagna) then it sense (feels) and then identify (percepts) . We say above mixing as impact or connecting (Sparsha). Then thinking about external masses with mind using thoughts - called as volitional states (Cetana). This sense and identify also help to create new formations in mind (Karma - Sanskara) while thinking process. After thinking, say as word from mouth or do actions from body or both. This thinking, saying & acting help to collections to stores in mind as energy packets called Kamma. Kamma is mental formations. 
          Sensing are mainly three types - positive sensing, negative sensing & neutral sensing. but with six senses it can be categorize as 18 types. as example eye sensing can be positive, negative & neutral. Identifying is six types - Identification from eye, ear, nose, tongue, body & mind. Collecting Kamma also stores in mind related to six senses : mental image, mental smell, etc... Six sensing, six identifying, collecting as Karma are result from impact. This process is widely described in dependent organism.
          Lord Buddha categorize five masses of clinging as : internal-external, heavy-soft, near-long, canny-uncanny, vanished in past-unoriginated future-originated present. As example if you lived in your past life as energy body gods then it was external, soft, long, canny & vanished in past. but your current human body is internal, heavy, near, uncanny & originated present. Buddha said all these characterizes five masses of clinging are unsteady, stressful & not-self.
  
  Example.. eye + light + eye consciousness - sensing from eye - identifying sensed object 
  - thinking related to  object
   
Real life.. eye + car + eye consciousness - sensing from eye - identifying car - thinking about car then create likeness related car via mind as "I also need such car". so negative Kamma stores in mind as lust about car.
  
Real life.. mind + mental image of temple + mind consciousness - sensing from mind - identifying temple as Buddhist temple - thinking about temple then mind become relax cause of temple as "Monks meditate in these kind of temple". so positive Kamma stores in mind as merits.  
  
Collecting Help To Rebirth
           Collecting Kamma energy give resources to rebirth. Buddha said " Mind is seed, desires are water and rebirth cycle is paddy field of massive bodies sensing, identifying & collecting Kamma; Stressful pains are crops." Creatures rebirth because they have Karma to finish. Good Karma gives luxury life while bad Karma gives pains. Enlighten person don't keep or collect Karma. Hence they are not rebirth. Creatures bond to rebirth cycle cause of 20 wrong ideas about five masses of clinging as steady & controllable soul. Those are : 

1-4. Creatures bond to wrong idea as massive bodies are soul, soul created from massive bodies, soul is inside massive bodies  & massive bodies are inside soul. 
5-8. Creatures bond to wrong idea as sensing are soul, soul created from sensing , soul is inside sensing & sensing is inside soul. 
9-12. Also as identifying is soul, soul created from identifying , soul is inside identifying & identifying is inside soul. 
13-16. Also as collecting Kamma is soul, soul created from collecting Kamma , soul is inside collecting Kamma & collecting Kamma is inside soul. 
17-20. Finally as mind is soul, soul created from mind, soul is inside mind Kamma & mind is inside soul. 
  
But we need to think massive bodies result from earth, liquids, heat & air elements. We need to think sensing, identifying, collecting Kamma is result from impacting. And mind is result from sensing, identifying, impacting, volitional states, recollecting and massive bodies (mind & matters or nama-rupa). 

Unknown ness
   We belongs to rebirth cycle cause of - Karma, bonds from desires and darkness of unknowingness. Also we cling sensed and identified things as My things. So wrong sensing & wrong identifying means we think all matters are immortal, pleasure & mine. But truth was all masses are destroy and change with time, therefore stressful and we can't control that changing means not ours (not mine). When we understand all "five masses of clinging" are not I & mine & my soul ( characteristic of anti-soul or not-self) then we are disgusted about  five masses of clinging. Then we will not cling to those. So without cling, collecting Karma & desires to world's things will not happen. Without Karma & desires, rebirth also not take place. So then that creatures become enlighten as wholesome right ideas. This was the main teaching & finding of Lord Buddha.