Hakim al Umma Mawlana Ashraf Ali Thanawi (Allah have mercy on him) said,

‘In regards to the issue of mawlid there is a delicate discussion that need not be mentioned to lay public (for it is difficult for them to comprehend it). It is that, people do it (mawlid) considering it to be an act of worship (ibadah). For it (mawlid) to be so (an act of worship) requires documentation (from Sunna, Fiqh). This documentation has not yet been established. Those who advice against it do so based on this.

The lay people do not understand this and subsequently conclude that these people stop them from remembering the Prophet (Allah bless him and give him peace).

(To elucidate further) Hakim al Umma gave the example that suppose if someone was to recite ‘Mohammed’ ‘Mohammed’ (Allah bless him and give him peace). Then it has to be investigated that is this (recitation) considered to be a act of worship or not? (If he considers it to be an act of worship) Then there is no proof of it.

Mazeed al-Majeed, page 13

This delicate point is indeed seldom appreciated, even by scholars.

The crux of all discussion to proof the validity of celebrating mawlid is on historic incidents. That is, such and such major Shaikh, grand Mufti or erudite scholar did it. These only establish the cultural events observed at that time.