The Melancholy of Mamoru(2006)

Mamoru-san no Yuutsu

Sakagami Mamoru works as an elementary school security guard with his beloved dog, Haru. Lately, a high schooler named Ayukawa Kyougo constantly seeks him out. Kyougo is teasing and forward, declaring that he’ll make Mamoru love him more than he loves Haru. However, Mamoru isn't convinced. He believes that, unlike dogs, human feelings are fleeting.

(Source: shinkeikaku)