It's been stated above, I agree...
It's not a lack of fear of death. I'd say it's a matter of having greater love of their friends. That's understandable.
Plus, as Star Fleet officers they have greater training in life-and-death matters, along with oaths to give their lives and all that.
Just to mess things up, I'll also toss in Kirk's line in TFF:
"I'm afraid of nothing."