Another source for Siyum Masechet is Shabbos 118b:
ואמר אביי: תיתי לי, דכי חזינא צורבא מרבנן דשלים מסכתיה עבידנא יומא טבא לרבנן
And Abaye said: May I receive my reward because when I see a Torah scholar who has completed a tractate that he studied I make a feast for the Sages.
If you notice, Abaye did not say that in his view, Sium was obligatory, or that he behaved that way, instead, he was arranging the feasts for other Sages, probably endorsing them to finish Masechtos and claiming the reward for himself.
If so, is the encouragement of having a feast, the only reason for having our Siyum Masechet tradition? In other words, only an external reason, nothing a learner is obligated intrinsically?
I understand that the Minhag is an accepted Halachah (Rem"o 246,26, thanks Avrohom), but as per his answer, no source in Shu"A brings reasons why, and how it follows Abaye.