many people are to shy to introduce themselves to people they to not know and start a conversation. British people usually feel uncomfortable if a stranger starts to talk to them,even while their are writing in a queue or standing at a bar when they know they can get away. But in the US to be outgoing (= eager to talk to others) is considered a positive quality.
Americans can be quick to make friends with people, but then may have so may friends that they cannot spend much time with them. Foreigners find Americans friendly but this cover that it is difficult to develop close friendships with them. British people, on the other hand, may same reserved (= rather shy and distant) at first but as they get to know some body they become more relaxed and a friendship may develop.
American may get on first-name basis (=use each others' given names) easily. Nearly all Americans under 50- friends, neighbours, co-worker, and business people-use each other's first name. In the US people are afraid of begin to formal, whereas in British, if people are not sure how to address somebody they call them 'Mr Richards', 'Mrs Davies',etc. rather than risk offence by using their first name.