Japji Sahib..the Premier Composition at the Beginning of SGGS..has been compulsory "reading" since the time of Guru Nanak Ji...Sikh History tells us that it was the recitation of Japji by Bibi Amro Ji, daughter of Bhai Lehnna Ji (Guru Angad Sahib Ji) that led her father-in-Law Bhai Amardass JI (Guru Amardass Ji) to Guru Jis DOOR...and later on become the THIRD GURU !!
(Btw in this Relationship - Bhai Amardass Ji was not only ELDER in age, he was also HIGHER in status as he was the Father of BibI Amro's husband while the GURU, Guru Angad Ji was Younger in age and "lower" in status as the father of the Bride !! BUT the two NEVER went into these highs/lows and nitty grittys and thus attained the same relationship Bhai lehhna Ji had with GURU NANAK JI !! This is a sign of TRUE Guru-Sikh Relationship as was between the first 3 Gurus rather than blood and social relationships which normal people pay much more attention to !!).
Anyway, the JAPJI being the PREMIER BANI has always ahd PRIME TIME importance in Sikhi/Gurmatt and REHRASS is also mentioned as a DAILY AFFAIR by Bhai Gurdass Ji about the Life of GURU NANAK JI and subsequnet GURUS..so it is CLEAR what NITNEM was followed and this NITNEM is CODIFIED by being inserted in the Beginning 13 Pages of SGGS - Selection of Shabads and TITLES - Jap and Rehrass given by GURU ARJUN JI.( This SELECTION is important because the SAME Shabads occur in their Rightful places in the various Raags later on in SGGS- so there is a METHOD to this and a REASON as to WHY the Selected Shabads are put in a Prime place and given titles. Its meant for us to concentrate on these 13 pages on a daily basis if this is the MINIMUM we can do. Thus its no surprise that even Bhai Nand Lal Ji in conversations with GURU GOBIND SINGH Ji stresses the PRIME IMPORTANCE Jap holds in a Sikhs Daily Life.
With such Clarity and method and reason given to us by the GURU in WRITING..we Sikhs still have a tendency to neglect this and go looking OUTSIDE for our own MANMATT.