In philosophy some fundamental experiences are not subject to discussion. Eg. I am hungry, I am cold etc. I could be faking it of course but for myself if I am cold I am cold!
Such experiences are called incorrigible.
Love is or at least comes close to being incorrigible. Yeah people will snarkily point to divorce courts as counter evidence! But love between parents and children comes closer:
Does your mother love you?
Do you need proof?
Can she give if you demand?
Love — whether one's mother's or God's — is not a meaningful subject for intellectual discourse and "logical proof"; you have it or you dont.
However the following analogy may show a way:
Analogy
A stool is stable on 3 legs. In the same way religious practice must engage mind, heart and body.
If youre a sikh this naturally expands as
Mind — Oneness of I-world-God
Heart — Love of God
Body — sewa to fellow beings seeing all as God only
You can translate sewa as service as long as the other two legs of the stool are not neglected!
Conversely, if the love leg is weak, the others can aid.
Added Later
Some Basic History
Sikhism follows the ten gurus from Guru Nanak to Guru Gobind Singh (1469-1708).
There was no strong demarcation between the sikhs and "normal" Hindus until the martyring of Guru Arjan Dev the 5th guru in 1606.
However sikhism in its modern distinctive sense only starts with the ninth and tenth gurus after the second martyrdom of Tegh Bahadur (1675) when the last guru Guru Gobind Singh militarized the community to resist the atrocities of the mughal tyrant Aurangzeb and expecting no guru successor, 'nominated' the Sri Guru Granth Saheb (SGGS) as the eternal 'guru' from then on.
This history is needed to understand that the major author of the fundamental book of the sikhs, Guru Nanak, was very much a Hindu religious reformer, and was not trying to start some fundamentally new system. This can be seen in the fact the names he used for God in the SGGS are the standard Hindu names, Ram and Krishna (Gobind and Hari). It's true that the form of worship is different, with (normal) hindus commonly using icons for worship, whereas sikhism disapproves of idols. This disapproval too is completely in line with traditional Hinduism where name is considered to be higher than form and both name and form are the lower forms of God with pure Existence-Consciousness-Bliss
being the higher form.
Once it is understood that sikhism is a reform movement within Hinduism, it can be appreciated that sikhism is in line with the primary Hindu teachings eg. of the Bhagavad Gita ie. (a) karma called sewa ie selfless action, (b) bhakti called prema is love, and (c) gyana is firm knowledge of non dual Oneness is the goal.
Path from service through love to wisdom/oneness with God
is described here:
This method of karam is acceptable, that which was instructed by Sri Bhagvan Krishna. By performing good actions —karam — one's mind becomes pure. Then a devotional spirit will reside within ones consciousness.
Sewa and prayer as a path to love
Just as a blind man may be cured by a suitable treatment a person without love can be cured by prayer and seva
SGGS Ang (section) 8
Sewa as a path to God
One who performs selfless service, without thought of reward, shall attain his Lord and Master.
SGGS
God IS Love
ie to love is to become God
Love is the Lord's Name, love is His form, love is His abode, love is His treasure.
Sukhamani Saheb from SGGS
Love is the Lord's essence, love is His form.
Asa di Var from SGGS