I am so sorry to hear that! Dogs are some of my favorite people -- full of life, enthusiasm and unconditional love.
Here is an e-mail I got once that I have saved because whether the story is true or not, the moral of it certainly is:
Or why i love dogs
Dog's Purpose (from a 6-year-old)

Being a veterinarian, I had been called to examine a ten-year-old Irish Wolfhound named Belker. The dog's owners, Ron, his wife Lisa, and their little boy Shane, were all very attached to Belker, and they were hoping for a miracle.

I examined Belker and found he was dying of cancer. I told the family we couldn't do anything for Belker, and offered to perform the euthanasia procedure for the old dog in their home.

As we made arrangements, Ron and Lisa told me they thought it would be good for six-year-old Shane to observe the procedure. They felt as though Shane might learn something from the experience.

The next day, I felt the familiar catch in my throat as Belker 's family surrounded him. Shane seemed so calm, petting the old dog for the last time, that I wondered if he understood what was going on. Within a few minutes, Belker slipped peacefully away.

The little boy seemed to accept Belker's transition without any difficulty or confusion. We sat together for a while after Belker's Death, wondering aloud about the sad fact that animal lives are shorter than human lives. Shane, who had been listening quietly, piped up, ''I know why.''

Startled, we all turned to him. What came out of his mouth next stunned me. I'd never heard a more comforting explanation.It has changed the way I try and live.

He said,''People are born so that they can learn how to live a good life -- like loving everybody all the time and being nice, right?'' The Six-year-old continued,''Well, dogs already know how to do that, so they don't have to stay as long.''

Prayers headed your way

My heart goes out to you and Mari. I can so relate to all the feelings you are going through right now. My husband and I bought our first dog in 1994 right before we got married and he was like our first child. He was just the best dog ever, so sweet and kind. In 2003, we found out he had cancer, multiple myaloma. He went through chemo for a period and was on prednisone for a long time. The doctors told us he would live 6 to 12 months longer. He went on to live for 3 more years. We treasured every minute of that time. I'm sending up prayers for you, Mari, Tyler, and for all your other family and friends who love Tyler as well. I know God will bless your time with him.

Rob, I am so sorry to hear about Tyler. I have lost both of my dogs (to illness) in recent years and miss them every day. I will keep you in my thoughts.

(((hugs)))

Rob,

So sorry you and your family have to endure this! I have been there myself with cancer and my dogs and now I am caring for my diabetic kitty. Tyler is lucky to have such a caring family. My thoughts and prayers are with you guys. I will be making a donation to PETS ALIVE as a tribute to Tyler. Peace and Love!

Jennifer

Ditto Doc Irene.
Not much else to say that she hasn't.
I'll be thinking of you all.

Oh my god...I'm really sorry to hear this...Here, near my chair, is sleeping my dog Paco..I look at him and I see Tyler, It's all so sad and i have no words to explain how deep this thing has touched me.
I understand how painful is for you to see Tyler in this state and I hope that these last months will be as peaceful as possible, though the situation is terribly unhappy.
I send all my Love to you all, Rob, Mari and Tyler.

So very sorry Rob and Mari. Enjoy every single second with your precious Tyler - as I know you already do. You will all be in my prayers and hoping for a miracle.

Rob and Mari,
It was so sad to come here and find this post. I am heartbroken for you guys. I wish Tyler nothing but the best. Dogs are such a special part of the family. When I had cancer and was going through chemo, my dog was right there with me, when everyone else was sleeping or busy, he never left my side. I know you guys will help him thoough this. Please take care and know that I am praying for Tyler.

My heart is breaking for you and Mari. You, Mari and Tyler are so lucky to have found each other!!! I know that you have all brought great joy into the other ones lives. That is such a wonderful gift!

Oh God, Rob I'm so sorry
When my cat was sick a few months ago, I just sat by her and cried for hours. She turned out being fine, but just the thought of her not being ok made me break down. I can't imagine what you're going through :'(
Give Tyler all the love in the world, lots of doggie treats, and tell him not to worry about that hair ok?
Take care,
Ariella

My thoughts and prayers are with you and your family. I lost my Yorkie 3 years ago on Valentine's Day and there isn't a day that goes by that my daughter, my family and I don't think about Maggie. Dogs are your children too and we are blessed to have them in our lives! Take care.

I am so sorry. I know how you feel as I lost my little girl (cat) of 20 years last year. I will keep you and your family in my prayers.
Rebecca

Rob - this breaks my heart. I am so sorry for you and Mari and Sammy - losing an animal is like losing part of your soul. Tyler is lucky to have the love of the two of you. You recently twittered that you should get up when your dogs wake you and I've taken that to heart. You are in our thoughts...

Hey Rob!
I am the most 'in love' with dogs person that i know!
And probably the weakest person if i'm to loose one of these beloved pets!

Just want you to know that my prayers go to Tyler!
God bless your family!

"Everything will be okay in the end...If it's not okay it's not the end"

I am so sorry about your baby, Tyler. We just went through the loss of our baby girl, Kiwi to Congestive Heart Failure. We did all we could to keep her comfortable in her last days, but the idea that she was dying all the time we had her at home just broke my heart. On July 6th her little heart just couldn't take it any longer and Kiwi went to join her furry friends that have passed before her. Our love to you, Mari and Tyler. We love them just like they are our children.