copy constructor const error while using with map

The following program works without error.

#include <iostream>
#include <algorithm>
#include <map>
using namespace std;
class userContext {
public:
    int id;
    int value1;
    int value2;
    userContext():id(-1),value1(-1),value2(-1){}
    userContext(userContext &context) {
        this->id = context.id;
        this->value1 = context.value1;
        this->value2 = context.value2;
    }
};
int main() {
    userContext a;
    a.id = 1;
    a.value1 = 2;
    a.value2 = 3;
    // map<int,userContext> Map;
    // Map[1] = a;
    cout << a.value1 << endl;
    cout << a.value2 << endl;
    return 0;
}

But If I introduce a map, it gives an error. Why is it so?

#include <iostream>
#include <algorithm>
#include <map>
using namespace std;
class userContext {
public:
    int id;
    int value1;
    int value2;
    userContext():id(-1),value1(-1),value2(-1){}
    userContext(userContext &context) {
        this->id = context.id;
        this->value1 = context.value1;
        this->value2 = context.value2;
    }
};
int main() {
    userContext a;
    a.id = 1;
    a.value1 = 2;
    a.value2 = 3;
    map<int,userContext> Map;
    Map[1] = a;
    cout << Map[1].value1 << endl;
    cout << Map[1].value2 << endl;
    return 0;
}

part of compilation error output:

locks.cpp:20:7:   required from here
/usr/include/c++/7/bits/stl_pair.h:292:17: error: ‘constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = const int; _T2 = userContext]’ declared to take const reference, but implicit declaration would take non-const
       constexpr pair(const pair&) = default;

However, changing copy constructor signature to userContext(const userContext &context) resolves the compilation error and programs executes fine. Please explain.

Thanks!