  Using NMTO-{\it downfolding} technique, we explore and establish the origin
of ferromagnetism in the pyrochlore system, Tl$_{2}$Mn$_{2}$O$_{7}$. It is
found to be driven by hybridization induced spin-polarization of the
delocalized charge carriers derived from Tl-$s$ and O-$p$ states. The
mean-field estimate of the ferromagnetic transition temperature, T$_c$,
estimated using computed exchange integrals are found to be in good agreement
with the measurements. We find an enhancement of T$_{c}$ for moderate doping
with nonmagnetic Sb and a suppression of T$_{c}$ upon application of pressure,
both in agreement with experimental findings.
