"Imagine it like this. The binding energy of tritium and deuterium is like a ball sitting on a table. The binding energy of helium is like that same ball sitting on the ground. Lifting the ball requires more energy if you have to lift it from the floor. Similarly dropping the ball gives you more energy if you drop it to the floor than you do to the table. Hence why fusing d-t together gives you energy, it is like dropping the ball from the table to the ground."