Love is a social state.
Dogs are social creatures.
Therefore it is likely that they experience a wide variety of social state, and dogs display all the characteristics of love including, sacrifice, loyalty, caring, comforting, joy, protection, gratitude, obedience, listening, communicating, friendliness and so on.
They certainly don't feel human love, they feel Doggy love, but it remains love nonetheless.
And of course the subjective or anecdotal view point of love...
If you don't feel love then they don't love you.
Therefore, If you feel the love, then they do love you.
Love is a two way street, and if the human loves the dog the dog loves the human.
I notice that there is a poster who wants to objectify all the various attributes of love down to food. You seem unaware that there are many other NEEDS of a dog, and specifically these including being part of a social structure known as a pack.
Love is a subjective state, one feels it. Either one feels it or not, it is highly unlikely that we will ever absolutely know that dogs experience love, but it is plainly obvious that they display a wide variety of indications they experience love and these indications are consistent across the species.
It's quite obvious to me on a variety of levels that dogs love.