With several billions stars in the Milky way galaxy and it now showing a fair percentage have planets there's likely to be at least a billion planets in our galaxy. Times that by over a billion galaxies. To say life only exists here is saying it was a 1 in 1, 000, 000, 000, 000, 000, 000 chance. Given life here on earth exists on land, in the sea, near poisonous gases, in the Artic, near volcanos, at high altitude and in acidic stomach acids, it's not like life even here is particularly restricted to an ultra narrow range of conditions. With evidence of water around exo-planets now coming in, whilst neither 'side' can claim an absolutes, those saying 'we are alone' are clearly making the larger leap of faith.Not being nasty or cynical, but 'anyone thinking we are alone' is merely making a conjecture based on multiple assumptions and an extremely small sample size of actual useful data - ie our solar system.
'Anyone thinking we are not alone' is doing exactly the same thing.