Comey admitted that a prior personal bias against Trump led to him write memos after meetings.

COLLINS: ... What was it about that very first meeting [intel briefing while Trump was President-Elect] that made you write a memo when you have not done that with two previous presidents? 
COMEY: ...  the circumstances, that I was alone [by decision of the intel leadership, not Trump], the subject matter and the nature of the person I was interacting with and my read of that person.