What do we need to change to make your dog want to work for food?

① The environment

② The food you are offering

③ The value of food

④ Emotional state

Not necessarily in that order, it depends on your problem. You may have to change just one of those, or a couple, or all of them.

Part 1: The Environment

If you are trying to get your dog to take food outside to work on lead walking, get a good recall, basic obedience like getting your dog to sit or down or even working with a reactive/fearful/anxious dog the environment matters.
Change the environment if your dog is too distracted by his surroundings
You and your food treats are competing against the environmental distractions.
Your dog will only succeed in an environment where it is possible for that to happen.
Low level environment -1 Your living roomHigh level environment – 10 Park full of dogs
Yes, the level 10 environment is the place you actually need this to work, eventually. 
But Rome was not built in a day.
People either try and train their dog in a level 10 environment which just won’t work as their dog’s head is full of everything else going on around him so he finds it impossible to concentrate on you or your food.
Or they practice in their living room with the tv turned off and no other people there then take the dog to the park and hit the dog up with a level 10 environment where he just can’t do it.
You need to introduce distractions gradually:
In the house put some of your dogs toys on the ground as a distraction while you train.
Put pots of food around so they can smell the food.
Ask family members to walk past while you practice
Put music on
Do it in a different room in your house.
Then try the garden at a quiet time of day
The back garden when neighbour is out in their garden
The garden when your neighbours dog is in her garden
Chat to your neighbour over the fence while you do basic obedience with your dog.
Front garden when there are noisy kids playing outside in the street.
An empty field or car park, a quiet country road
A park with a kids playground a distance away
A park with a person and a dog right at the other end of the park
A quiet residential side street
A busier street
A park with a few dogs
Slowly, slowly build distractions. Give your dog a chance to be able to succeed at the task by gradually stepping up the level of distractions.