Hello, I'm new to calorie count...

I joined not to lose weight, but to ensure that i am intaking enough calories to maintain my body mass, or increase it.  I'm a medium frame, athletic and active male.  Really, I would like to gain more muscle mass.  With a high metabolic rate, this proves to be difficult.  I am working out as time permits, and hopefully eating enough protien to get to a healthy 155-160 lbs.  I am currently 145 lbs.  So, I've got some work to do.

Question:  Should I try to gain mass?  I am healthy, active and fit now.  But I would like to have larger muscles in general.  If I achieve this goal to be 155-160 lb., what do you think this would do to my overall life span/general health?  Shorten/worsen it...?  Am I better off just being the weight God intended, or going againts the grain?

Input appreciated-Thanks!