When some one rises to call others towards Allah , His act will not only benefit others but more to Himself and his family Members.
So Inviting others to Islam is an self improvement Process in other words and that way we are not obliging Allah (Subhanahu wa Ta'ala) but actually our self only by coming on to righteous.
As Allah says;
"But among you are those who are niggardly, and whoever is niggardly is niggardly against his own soul; and Allah Ta'ala is Self-sufficient and you have need (of Him), and if you turn back He will bring in your place another people, then they will not be like you."
[surah 47:38]
Some changes that are oftenly seen as sign of improvement in Dayi' (caller)
1. Knowledge
This is fruit of Da'wah as one who involves Himself as Dayee will increase his knowledge for benefit of others but actually this is benefit for him too in terms of this world and also life after.
2. Character
Dawah effects onto character of person as more and more knowledge of Quran and prophetic Traditions will Improve positive and slowly remove negative features in person. This is due to self realization and comparing himself on edge of Qur'an and Hadith
3. Taqwah
Taqwah means fear of Allah, more learning and saying will benefit person in acquiring more pity in Heart due to an inner feeling of implementing the same in his life rather then saying it only to others
4. Attitude
Dayi will slowly develop an beneficial Attitude that is calling people with wisdom and politeness and not to impose his views, that can help him to develop preferred personality among others, even in people who do not agree with his Beliefs will appreciate his attitude
In another way this will also be Helpful to him as his trust on Allh (Subhanahu wa Ta'ala) will increase that way
5. Satisfaction
This is a natural gift can never be acquired by wealth or any other means and most of people lack it . This is the main cause of all sort of instability and crises but most of people do not know
6. Steadfastness
The best according to me is that a Dayee’s efforts will make him steadfast on the way of Allah Talaha and Traps of shaitan will not harm him in great sense