iPhone 5 always asks for username when connect to Wifi (I'm using wifi from a personal access point in my room, of which the security type is "WPA/WPA2 Personal" (it's what I see in network settings on my mac). Normally it just requires a password to login, no username is required.

My issue is that when I use my iPhone 5 (iOS 7.0.4) to connect to that wifi, it always asks for both username and password. I tried leaving the "username" field blank and fill in the "password" field only, but it says "Unable to connect to...".

This issue happens only on my iPhone and doesn't happen on any other devices such as Macbook, iMac or iPad.
The problem is that iOS is detecting your network as an Enterprise network that requires username and password. 
The work-around:
Manually enter the network type into iOS. To do this with iOS 7.1:
Settings / Wi-Fi / Choose a network, and select "Other".
Put in the name (SSID) as it is advertised on the previous screen.
Under Security, choose WPA2 or WPA (try both). Do not choose the "Enterprise" network. Enter the password.
My experience is you have to try 3 or 4 times before it works; don't know why, maybe fat fingers.