I believe it's a phase, and a personality thing. My DD went through it in a big way, but she was always quite shy, she took a very long time to get used to people and open up. Now that she's 3.5 she's very outgoing.

DS is 15 months, and he's always been a very open kid, happy to go to anyone. He is a bit more wary of strangers at the moment, and will run at me with his arms up if he's unsure of someone. He's giving out the hugs and kisses a little less freely lol. If he's anything like DD though, I expect it will pass.