せる

Etymology 1
From Classical Japanese, which followed.

Suffix

 * 1)  to make ... do ...

Usage notes
Attaches only to the of  verbs and the irregular verb. The result is conjugated like an verb.

The passive form of verbs from other conjugation classes are formed with instead of.

In colloquial speech, can be contracted to, which follows.

In traditional Japanese grammar, this is a. Morphologically, this is an inflectional suffix whose proper shape is -ase- (terminal form -aseru), attaching to the stem of consonant-stem verbs.

Etymology 4
The attributive conjugation of stative, in turn analyzable as + stative auxiliary suffix. See for further details.