Setting this inside the functions Javascript

I have the next code:

const obj = {
    name:  'Hi',
    first: () => {
        return this
    },
    other: {
        name: 'last',
        sec: function() {
            this.c = '2'
            return function() {
                this.s = '3'
                return this; // expect {s:3}
            }
        }
    }
}


const t = obj.other.sec()
console.log(t())

In console.log i expect {s:3} but there is not that object.
Question: Why there is not {s:3}? Or function() should have its own context