Why is ionization energy positive?

Hint:The ionisation energy is a type of enthalpy which is required to remove an outermost electron from an isolated gaseous atom.
The value of ionisation energy is higher in case of positive ions or cations as there is already one less electron which makes the other remaining electrons more attracted towards the nucleus and so it is difficult to remove another.

Complete step-by-step answer:Ionisation energy could be defined as the energy or enthalpy required to remove the most loosely bound electron from the outermost shell of an isolated gaseous atom. The value of ionisation energy increases if the effective nuclear charge experienced by the outermost electrons from the nucleus of the atom, becomes higher. So, this reason explains the trends we get to see in the periodic table regarding the ionisation energy values of the elements.
As we go down the group, the ionisation energy decreases as with each progressive element, one shell is added to the element and so the effective nuclear charge experienced by the outermost electrons becomes less, which makes it easier for us to remove the electron using lesser energy. Similarly, when we move across a period the nuclear charge increases and so it becomes difficult for us to remove an electron and so it requires more energy to pull it out, hence the ionization energy increases.
The value of ionisation energy is positive because we are giving heat to the system in order to carry out the process of removal of electrons.

Note:There is a trend regarding ionisation energy of elements in the periodic table, which relates to the size and effective nuclear charge experienced by the electrons in the outermost shell of the elements.
The ionisation energy of the negative ions is lower than that of positive ions as, the size of the anions becomes bigger after addition of one electrons and so it is easier to remove the electron from it, as compared to the cation whose size is smaller because of the removal of one electron.