While Matsuda got along with the others for the most part, there was one (with the exception of L) he got along with a lot. Being roughly the same age and liking the same things, it didn't take a genius to figure out why Matsuda enjoyed spending company with him.
Ukita.
Ukita and Matsuda were a lot alike. Both were single, both guys, both lovers of cheap ramen noodles. Both lived like college students, and both were extremely dedicated to their jobs. They weren't sure who the older one was, and didn't really bother to ask. It was simpler to keep age out of the matter, they figured.
If Ukita and Matsuda were alike in so many ways, it seemed that L shared one quality with Ukita. That being they both harbored a crush on Matsuda. And like the other, it was perfectly obvious the other was in love with Matsuda but couldn't really understand that they were in love with Matsuda too. For a supposed genius, L sure was slow when it came to his own feelings. They would both fight over Matsuda's attention in their own childish ways - L would have Matsuda help him look something up, and Ukita would come and insist that he didn't understand what he was doing, or that they should go on a break. Ukita would sit next to Matsuda on the couch, and L would worm his way between them and eat his cake and talk with Matsuda, completely ignoring Ukita.
However, even L realized that in the long run, Ukita would win Matsuda in the long run. Ukita went out and did things; it was him who discovered that Matsuda loved festivals and anything that got people together to do things. And if Matsuda was faced with the decision to go sing karaoke with Ukita or stay and work late with L, even the Chief's young daughter of average grades could figure out what Matsuda would pick.
And so, L unconciously vowed, he would do his best to keep Matsuda's interest. Permanently.