I first found it difficult to know what angle to approach this from because being grateful and giving thanks is so often thought of in a religious context, and I have no religion. If one is not giving thanks to a higher being or some other similar outside influence, it can be hard to explain who or what one is thankful to for the things that one is thankful for.
Sometimes it's obvious to whom one is thankful.
There are small things - for example, today I received a gift of knitting notions, and of course I thanked the gift-giver for her thoughtful and generous present, chosen for me and sent halfway around the world.
There are the bigger things - I am grateful to my parents for raising me as they did, for giving me the opportunities that they have, and for encouraging me in whatever pursuits I chose to take up. I am grateful to my husband for choosing to share his life with me and for the team that our marriage makes us.
There are times when the one who receives our gratitude is unknown, or anonymous, or even a stranger. I will be forever grateful to the surgeon who saved my life when I had life-threatening complications after the birth of my older son. I did meet him, once, the following day. To him I was probably just another patient, another case dragging him out of his bed in the middle of the night, a body on an operating table, and I don't expect to be any more, but I am very glad he did what he did that night. Even more anonymous will be the debt of gratitude I owe to the ten people whose donations of blood were used. I used to give blood myself - the transfusions mean I'm no longer allowed to - and it wasn't something I really thought much about from the donor point of view, but as a recipient, strangers really are one reason I'm still here today.
Then there are the things we can be grateful for that we can't control. Those are the things that a religious person might attribute to faith or prayer.
Again small things - a sunny day for an outdoor celebration, not too much traffic when you're running late and in a hurry, having just enough jam left in the jar for that morning's toast, and bigger things - being born in a country with freedom and democracy, into a family with the means and will to take proper care of me, having the right to a good education to enable me to find a good job and take advantage of all the benefits that go along with gainful employment, having met a good husband and being able to have two beautiful children.
When you don't have a religious target for your gratitude, all you can do is call it luck. Coincidence. Being in the right place at the right time. There's not much point expressing gratitude to that because it didn't choose to help you in the first place. Luck isn't personal, it isn't something you've earned or worked for. The nastiest people can also be very lucky; the kindest and most deserving are sometimes the ones that everything bad seems to fall upon. All you can do is take the chances you get, make the best of what does happen to come your way, and be glad when good luck does befall you, even if there's no one to be grateful to specifically.