You are:
THRILL
From hunting to movie watching, I'm sought out,
The thrill of a hunt, movies that are thrillers
Even on birthdays or Christmas day, there's no doubt.
The thrill of gifts on these occasions
Envied by many, who live a boring life,
They'd love to have some thrill to spice up their life
Let's be careful though, I can bring you much strife.
Thrill-seeking behaviour can be risky/dangerous to life and limb
In cases of life and death, I'm there the most,
Near-death experiences give the biggest thrill
Never dull, nor fearful, I strengthen my host.
Thrills are the opposite of dull, thrill-seekers are not fearful/timid
Thrills release adrenalin, which gives the host a boost of energy (the flight/fright/fight syndrome)
Gone as fast as I come, but I'll come again,
The thrills come as a rush, and dissipate after some time
So don't be a junkie, 'cause you'll see me then.
Don't become a thrill-seeking adrenaline junkie