There are some that claim there is no such thing as a "healthy addiction" - others choose "feeling healthy", or "being loving" as healthy addictions.
I suppose it depends on how you define addiction, and, how you define healthy.
"Anything that fulfills 4 of the 6 human needs at high levels, you are addicted to" - Tony Robbins.
So, for example, your wife who you are certain loves you and will be there for you, who you feel highly connected to, enjoy contributing towards who is constantly surprising you with her zany antics... congratulations, you're addicted. now just imagine if she also challenges you to grow, and occasionally compliments you? Well dang son, you've got yourself a full fledged addiction.
In my "happy and fat or thin and miserable" article, I mention the benefits of finding an exercise you can enjoy.
Let's expand upon that a bit to drive the point home:
Certainty that you'll get a great workout, have fun, and learn something: High
Connection with classmates and instructors: High
Contribution with newer classmates (this one takes a bit before you feel comfortable teaching, but, eventually...) High
Growth of character, ability, aptitude, and over all strength/speed/conditioning: High
Uncertainty as to what you'll learn, how, and what you'll be doing that class: High
Significance you feel from your sense of accomplishment, overcoming personal boundaries, and helping others: High
Addiction level: 10/10
Now let's compare that with "going on a diet and going to the gym for a month to lose a few pounds" mindset.
Certainty that it'll be hard work, that the gym will be open: High
Connection from making friends with regular gym goers: Low (generally it's new people every time)
Contribution from helping others work out: Low, and creepy.
Growth from overcoming personal boundaries, and increasing fitness levels: High
Variety of work outs: Low, there's only so many machines.
Significance of lookin' good, and pride from sense of accomplishment: High
Addiction level: 5.6
Check out "Fun Exercise", and try some out. You might find yourself healthily addicted within no time ;-)