Guy's Behavior

Why did he sound sad over the phone?

Been seeing this guy for a while but we're nothign serious. We do hook up and talk a lot when we see eachother, we're kind of like FWBs I guess. Anyways I couldn't hang out last week because I've been really busy with work, and he cancelled plans last weekend because of the same so it's been a couple weeks. Last night he got in touch asking if he can see me tomorrow. I told him I'm busy and to call me tomorrow afternoon. So he just called and asked how works been and tried to start up a little conversation and then talked about seeing me tonight. For some reason though he sounded like he was really sad. I wanted to ask why he does but I didn't and spoke normally and kept the call short. Why did he sound like he was sad when he called?
Why did he sound sad over the phone?
Post Opinion