Ever been asked?
What would you say?
I think as a young person I would have delivered a conversation stopper:"Because I am saved from my sins by Jesus." and made part of his family for ever...if I wanted to extend it a little.
All true, but not guaranteed to promote conversation with the questioner.
If asked today, I'd first ask 'why do you want to know?' If they say they are curious, or similar, perhaps enlarge on their curiosity. The enlargement can be either genuine, or not, and either convivial or oppositional; that is, to set you up for ridicule.
Taking all that into consideration I'd answer: 'because the Christian story of reality is comprehensively descriptive of life and how we experience it.' [its more than this, but, as in sales, you don't deliver the whole story like a recitation, you use the conversation to build a natural relationship].
Then you might have opportunity to discuss how God ‘out-graces’ our wrecked lives and gives us restored relationship with him replacing our disrupted human condition with peace, joy and deep satisfaction of new life in God’s family.
Or, briefly: because its true (in how it describes life and our relationship to reality).
If they are aggressive in deprecating you, there's either a friendly response, or a 'tough' one.
Friendly: 'are you so aggressive because your own world view won't stand up to real life/is too challenged by mine?'
Tough: 'so I guess you're an atheist or materialist, am I right? Or maybe into scientism? [person assents]. Then how is your world view anything worth even thinking about: a trivial meaningless life full of sh*t and then you turn to dust'
Rarely a person will not be in the materialism squad and will be a 'new ager' or a mainstream Eastern spiritualist. Both deny the final reality of the person, believe in an impersonal spirit, have a diffuse view of creation and no room for beauty, joy or true value other than contingent transient reactions, of the 'I like icecream' variety.
**Scientism is the usually informal or unconscious belief that the physical sciences are exhaustively and entirely descriptive of reality.