My brother put his 18-year-old dog to sleep yesterday.

My sadness today is profound, almost overwhelming, and I am trying to figure out why.

Obviously, the dog himself, a terribly sweet, ridiculously cute cocker-beagle mix, is the primary reason. He was my brother's dog -  there's no denying that - but he was really my first pet as well, my roommate and companion for the nine-plus years I lived with my brother after college.

When I came home from my first real job, he would greet me with that wagging stub of a tail and the butt jerking uncontrollably from side to side. I would lie on the floor, and he would pin me down, licking my face til I could stand it no longer.

I took him for walks every day. I taught him roll over - a trick we had to retire several years ago when it became too demanding for his aging frame - and play dead - which he did pretty well, except for that dang wagging tail, which couldn't help but anticipate the forthcoming treat.

Lucky gave my life joy and meaning, structure and responsibility.

However, I moved out on my own five years ago, and while I saw Lucky at least once a week and would occasionally watch him when my brother left town, I was no longer much of a caretaker for the dog.

It was my brother who really had to put up with Lucky's growing eccentricities - like the way he would whimper for hours on end and his increasingly picky appetite (a sure sign of sickness as this was a dog, after all, that would once eat the grossest things the New York City streets had to offer) - and who near the end had to give him the daily injections of IV fluid and clean up all the household accidents as his kidneys started failing more rapidly.

So while some of my connection to Lucky might have been lost over the years, I'm sure some of my sadness also stems from how intensely I feel my brother's loss. I was there with my brother as he made the correct but horribly final and painful decision to give Lucky a peaceful end, and as he held the dog's body in his lap one last time. And at least some of my pain and sadness must stem from knowing how badly my brother is hurting right now.

And I think there is something else that is making me sad. Something a bit more esoteric, a bit more selfish, and yet just as deeply felt: Lucky's death in a certain way marks the passage of an era for me. I first met that dog when my brother, who had adopted Lucky a few months earlier, picked me up at the San Francisco airport when I moved there after college, armed with nothing more than a suitcase full of clothing and a journalism degree from Northwestern University. It was such an exciting time. My life and all its wonderful possibilities seemed ahead of me.

And for the next decade and then some, from one coast to another, from one job to another, Lucky was a part of that growing-up experience. It's been fascinating to see all the people who've been part of my life the past 14 years - high school and college friends who came to visit, new friends, co-workers and colleagues, family members I got to know for the first time - who met Lucky and felt compelled to express their own connection to him on Facebook.

All those people who have been in and out of my life, and all those days, it seems to have flown by in an instant, and I wonder sometimes if I've made the right decisions in my life, if I've taken full advantage of the opportunities given me, and whether i am happy with where I've ended up.

Yes, I am married with a great wife, have my own awesome dog and am expecting a baby daughter in the fall, and I know that challenging and exciting moments are ahead of me. But that special post-college time - when my life and its direction seemed a complete mystery, even to me - feels like it now has passed forever along with Lucky.

Yesterday, my brother, his girlfriend, her sister and I took Lucky to the park where he had spent so many happy moments. It was such a beautiful day, with a bright sun and mostly cloudless sky giving off the gentle warmth of early spring. Lucky seemed very happy, taking in the familiar smells, feeling the soft grass beneath his paws, enjoying all the extra attention he was getting (though I'm sure all of the petting was a bit uncomfortable on his sore body, he took it like a champ, there for others until the end.)

Keenly aware of how easily we can take time, and loved ones, for granted, I told myself repeatedly to appreciate these moments, absorb them fully, take it all in, the beauty of the day, the pain of the impending loss. We would never have it back. Not the dog, not the day, not the emotions. None of it.

Now, as I sit here less than 24 hours later trying to recapture those moments, the memories are already fading. Pictures are blurred, hazy, insufficient.

And if that isn't a reason for profound sadness, I'm not sure what is.