Question: Why don't Sikhs actively proseltyse like Muslims?

Below is a response to a Muslim gentleman who asked why don't Sikhs do "Dawah", i.e. publicly claim Sikh to be the Truth and invite others to join the faith, like Muslims do?

RESPONSE:

Gurbani says:

ਬੀਰ ਦਾਵੈ ਦਾਝਨੁ ਹੋਤੁ ਹੈ ਨਿਰਦਾਵੈ ਰਹੈ ਨਿਸੰਕ ॥ 
O Kabeer! Those who brag with claims, shall burn within. Those who do not brag with claims remain care-free. 

ਜੋ ਜਨੁ ਨਿਰਦਾਵੈ ਰਹੈ ਸੋ ਗਨੈ ਇੰਦ੍ਰ ਸੋ ਰੰਕ ॥੧੬੯॥ 
That humble being who does not brag with claims, looks upon a king and a poor person alike. ||169|| 
(Salok Kabeer Ji - Sri Guru Granth Sahib Ji - Ang 1373)

A Sikh is not obligated to make claims that he or she knows the Truth or that they are saved in the hope of others converting. A Sikh is to remain humble and free of ego. When one brags and claims they have achieved the Truth etc, it is easy that they will get distracted from their own spiritual path and instead of getting closer to Vahiguru, get closer to feeding their ego.

The Sikh Gurus never pursued to mass convert people to make numbers. In Sikhi, quality is more important than quantity. Merely, getting someone to acknowledge or accept Sikhi as the Truth, as other religions do, does bring salvation. Guru Ji says:

ਬਿਨੁ ਕਰਤੂਤੀ ਮੁਕਤਿ ਨ ਪਾਈਐ ॥
Without actions or effort, liberation is not obtained.

ਮੁਕਤਿ ਪਦਾਰਥੁ ਨਾਮੁ ਧਿਆਈਐ ॥੧॥ ਰਹਾਉ ॥
The wealth of liberation is only obtained by meditating on the Naam, the Name of the Lord. ||1||Pause||
(Gauree M:5 - Sri Guru Granth Sahib Ji - Ang 201) 

Changing someone's religion through debate could be quite easy, however, changing someone's behaviour, habits and how they are as a person within is difficult. People brag that they have converted so many people or there faith group is growing in numbers, however Guru Ji says, being a true Muslim is very difficult.

ਮੁਸਲਮਾਣੁ ਮੋਮ ਦਿਲਿ ਹੋਵੈ ॥
O person of God! An actual True Muslim is one who has soft kind heart,

ਅੰਤਰ ਕੀ ਮਲੁ ਦਿਲ ਤੇ ਧੋਵੈ ॥
and one washes away the inner pollution (of evil desires) from within their heart.

ਦੁਨੀਆ ਰੰਗ ਨ ਆਵੈ ਨੇੜੈ ਜਿਉ ਕੁਸਮ ਪਾਟੁ ਘਿਉ ਪਾਕੁ ਹਰਾ ॥੧੩॥
(That True Muslim) does not even go near the worldly pleasures, and lives with purity just like flowers, silk, ghee and deer-skin remain pure. ||13||
(Maaroo M:5 - Sri Guru Granth Sahib Ji - Ang 1084)

Those who Dawah and preach to others, must look within and ask if they have achieved being a True Muslim first.