Some possible reasons

1. The Celtics are an Atlantic division rival that is one of the most storied franchises in NBA history. While the Celtics have a long history of winning, the Raptors have a history of losing ... until recently.

2. The Celtics have had little trouble with the Raptors over the years, with a 50-35 head 2 head record. Any fan following the NBA between 2006 to 2010 would remember the routine beatings the Celtics would give the Raptors with guys like KG, Pierce, Allen, and Rondo. The tables have turned a bit recently.

3. Despite the Raptors fielding possibly their best team ever, the Celtics are still higher in the standings and arguably better (depending on who you ask)

4. Recent comments were made between IT and Valanciunas about seeding. Valanciunas said the Raptors are competing for first place while the Celtics are fighting for second.When the Celtics eventually passed the raptors in the standings, IT responded by saying "words come back to bite you if you can't back it up"

5. Jae Crowder's 2016 comments on Raptors: "Toronto is not a team we’re worried about. I think Cleveland is the top team. That’s what it comes down to"

6. The Celtics have a strong association with Paul Pierce, recent Raptor killer