Why are some believers deeper than others?
Because they have decided to follow the Lord and have to held that decision.
Christian character is evident in the life of those who refuse to dabble in questioning God. We have His Word and the person of the Indwelling Holy Spirit. It is not possible to go back - to wriggle out of salvation. Either is truthful or He is not.
Depth is not part of the personality. It is the fruit of believing God and living accordingly. Those who live by sets of rules do not understand rest in Him and have no experience of the Everlasting Arms. Consequently, they deal with life on a surface basis, including measuring themselves by the behavior of others.
The deeper, the simpler. The more reliance on Him. The more love for His Body. The more clinging to His Word. Less judgment. More grace.
The deeper, the more like Jesus.