Why does solid snake vomit in Metal Gear Solid 4

Snake vomits in Metal Gear Solid 4 when the player kills a large number of enemies in conflict zone (Act 1 and Act 2). Before Old Snake vomits you will hear Liquid from Metal Gear Solid say "You enjoy all the killing, that's why!". Note: You can delay the vomiting by 'hugging' a wall. The next time you kill someone, however, it will happen again until Snake finally vomits.



Also why he vomits is if you take too much of the syringe or if you roll in the drum can for a long time.