A high school senior from Arizona made his classmate's day with a simple gift: a toy cement truck, covered in Christmas wrapping paper.

Maricopa High School senior Jake Marr decided to buy the present for his friend, Matthew Sabetta, who has a rare genetic condition called Williams syndrome, CBS Phoenix affiliate KPHO reports.

Sabetta's condition means he is missing 26 to 27 genes. But his grandmother says he was given something extra in return – what she calls the "joy gene."

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Sabetta and others with Williams syndrome have infectious happiness, and people like Marr appreciate this uplifting mood. The two became friends after meeting at school. Now, they regularly chat in the hallways before and after class.

To Marr, Sabetta's friendship is a gift every single day. So, he wanted to get his buddy a gift in return. He learned that Sabetta likes cement trucks, so he bought him a bright yellow toy one.

Marr walked up to Sabetta with the wrapped box as a friend filmed the moment. Before Marr could even hand him the gift, Sabetta started shouting in excitement. "Oh my God!" he yelled. He began tearing into the paper and exclaimed, "What's in here? I can't wait!"

Jake Marr bought a present for his friend Matthew Sabetta, and the video went viral. Jake Marr

When he realized his friend bought him a cement truck, he was overcome with joy and gave Marr a huge hug. His reaction was priceless and it went viral online.

"It was a big surprise," Sabetta told KPHO. "I got happy tears. So it was just my lucky day."

Marr's friendship is important to Sabetta. "He's my best friend, right here," he said he stood by Marr's side, clutching his new toy,

This friendship is important to both high school boys. KPHO

Their friendship taught Jake something important, too. "Just be kind, talk to them, get to know them, they have the most amazing stories you'll ever hear," he said.

Sabetta may have been the one to get a Christmas present, but seeing the smile on his friend's face was a big gift to Marr.

"Because he makes everybody's day, I figured I can make his day at least once," he said.