Dispassionate describes someone who is not getting carried away by — or maybe not even having — feelings. It's something you'd want to see in a surgeon, who keeps cool under pressure, but not in a romantic partner.
Dispassionate is the opposite of passionate, and while passions are said to run "hot," dispassionate people are often described as "cold." Many people think that a judge should be dispassionate when deciding a case — unaffected by strong emotions or preconceived prejudices.