An avreich spent a few years working towards his goal of learning the entire shas. His plans for celebrating the monumental achievement did not include anything more than having a schnapps and saying lechayim, and getting back to learning. His friends were pushing him to make a nice party, commensurate with the level of accomplishment, though he considered that bittul torah and too much of a bother..

This accomplished avreich went to Rav Chaim Kanievsky to ask for guidance as to what type of siyum he should have.

Rav chaim Kanievsky considered the matter and told him that while he is not obligated to throw a big party, it is appropriate for him to make a respectable seudas mitzva, and not just something small to be "yotzei". Rav Chaim explained that others seeing a respectable seuda for a siyyum causes an increase in their hearts the love of Torah and will cause them to strive to complete shas as well. Being that such a siyyum would be an encouragement for other people, it is worth doing.
source: Actualic