Drax looks out over the tranquil landscape and in an uncharacteristically calm manner states that it reminds him of taking his daughter to the lake. Mantis wanting to share this happy memory that can calm the usually boisterous Drax, reaches out and touches his arm. She then breaks down in tears.
Drax was being played as comic relief in this movie to the point that you had forgotten his back story and may have felt that he had too. It turns out that he hadn't and the revelation that such a happy memory is tinged with unbearable pain reveals a lot of depth about his character. He failed his family. They are dead because of him. He may carry himself as a bumbling idiot but he is driven to make amends for this failure. Maybe to even prevent it from happening to others. His boisterousness is possibly just him putting on a mask to hide the pain that he still feels inside.
It would have been so easy for this scene to have been cut. To leave Drax as just this lummox and comic relief. We know from this scene that Drax remembers and that he will protect his new family to prevent what happened to his old family. This scene says so much with very little dialog.
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